I am having the same error opening Tekkit Here is the full message ( I know its not a spoiler but if I just typed it out it would be the longest post ever 
)
44 mods loaded
Optifine OptiFine_1.2.5_HD_S_C3
Minecraft Forge 3.3.7.135
FML v2.2.48.135
Forge Mod Loader version 2.2.48.135 for Minecraft 1.2.5
mod_CodeChickenCore : Initialized (modpack.jar)
mod_MinecraftForge : Initialized (modpack.jar)
mod_NotEnoughItems : Initialized (modpack.jar)
mod_ReiMinimap : Initialized ([1.2.5]ReiMinimap_v3.2_03.zip)
mod_IC2 : Initialized (industrialcraft-2-client_1.97.jar)
mod_IC2AdvancedMachines : Initialized (AdvancedMachines_4.0_client.zip)
mod_BuildCraftCore : Initialized (buildcraft-client-A-core-2.2.14.zip)
mod_BuildCraftBuilders : Initialized (buildcraft-client-B-builders-2.2.14.zip)
mod_BuildCraftEnergy : Initialized (buildcraft-client-B-energy-2.2.14.zip)
mod_BuildCraftFactory : Initialized (buildcraft-client-B-factory-2.2.14.zip)
mod_BuildCraftTransport : Initialized (buildcraft-client-B-transport-2.2.14.zip)
mod_AdditionalPipes : Initialized (buildcraft-client-DA-additionalpipes-2.1.3.zip)
mod_ComputerCraft : Initialized (ComputerCraft)
mod_CCTurtle : Initialized (ComputerCraft)
mod_CraftingTableIII : Initialized (CraftingTableIIIB1.8.zip)
mod_mocreatures : Initialized (DrZharks MoCreatures Mod v3.6.2.zip)
mod_EnderStorage : Initialized (EnderStorage-Client 1.1.1.zip)
mod_EnderStorage : Initialized (EnderStorage-Client 1.1.3.zip)
mod_IC2NuclearControl : Initialized (IC2NuclearControl_client_v1.1.9b.zip)
mod_ImmibisCore : Initialized (immibis-core_49.0.7_for_1.2.5-client.jar)
mod_ImmibisCore : Initialized (immibis-core_49.1.0_for_1.2.5-client.jar)
mod_IC2 : Pre-initialized (industrialcraft-2-client_1.95b.jar)
mod_InvTweaks : Pre-initialized (InvTweaks-1.41b-1.2.4.zip)
mod_MAtmos_forModLoader : Pre-initialized (MAtmos_packaged.zip)
mod_ModularForceFieldSystem : Pre-initialized (mffs_rev3_for_1.2.5-client.zip)
mod_IC2_ChargingBench : Pre-initialized (mod_chargingbench-client-1.95b-1.2.5-r1.zip)
mod_CompactSolars : Pre-initialized (mod_compactsolars-client-2.3.2.10.zip)
mod_IronChest : Pre-initialized (mod_ironchests-client-3.4.2.28.zip)
mod_RedPowerCore : Pre-initialized (RedPowerCore-2.0pr5b2.zip)
mod_NetherOres : Pre-initialized (NetherOres_Client_1.2.1.zip)
mod_PowerConverters : Pre-initialized (PowerConverters_Client_1.3.4.zip)
mod_Railcraft : Pre-initialized (Railcraft_Client_5.2.4.zip)
mod_RedPowerControl : Pre-initialized (RedPowerControl-2.0pr5b2.zip)
mod_RedPowerLighting : Pre-initialized (RedPowerLighting-2.0pr5b2.zip)
mod_RedPowerLogic : Pre-initialized (RedPowerLogic-2.0pr5b2.zip)
mod_RedPowerMachine : Pre-initialized (RedPowerMachine-2.0pr5b2.zip)
mod_RedPowerWiring : Pre-initialized (RedPowerWiring-2.0pr5b2.zip)
mod_RedPowerWorld : Pre-initialized (RedPowerWorld-2.0pr5b2.zip)
mod_TubeStuff : Pre-initialized (tubestuff_49.0.7_for_1.2.5-client.jar)
mod_TubeStuff : Pre-initialized (tubestuff_49.1.0_for_1.2.5-client.jar)
Balkon's WeaponMod : Pre-initialized (WeaponMod.zip)
mod_WirelessRedstoneCore : Pre-initialized (WR-CBE Core-Client 1.2.2.1.zip)
mod_WirelessRedstoneAddons : Pre-initialized (WR-CBE Addons-Client 1.2.2.2.zip)
mod_WirelessRedstoneRedPower : Pre-initialized (WR-CBE RedPower-Client 1.2.2.1.zip)
Minecraft has crashed!
----------------------
Minecraft has stopped running because it encountered a problem.
--- BEGIN ERROR REPORT 5f97ded2 --------
Generated 04/07/12 16:26
Minecraft: Minecraft 1.2.5
OS: Windows XP (x86) version 5.1
Java: 1.7.0_05, Oracle Corporation
VM: Java HotSpot™ Client VM (mixed mode), Oracle Corporation
LWJGL: 2.4.2
OpenGL: ATI Mobility Radeon X1400 x86/SSE2 version 2.0.6068 WinXP Release, ATI Technologies Inc.
java.lang.IllegalArgumentException: Slot 249 is already occupied by ic2.common.BlockTex@1ac1928 when adding ic2.common.BlockTex@1a7c53e
at pb.<init>(Block.java:251)
at pb.<init>(Block.java:284)
at ic2.platform.BlockCommon.<init>(BlockCommon.java:12)
at ic2.common.BlockTex.<init>(BlockTex.java:30)
at mod_IC2.load(mod_IC2.java:154)
at cpw.mods.fml.common.modloader.ModLoaderModContainer.init(ModLoaderModContainer.java:324)
at cpw.mods.fml.common.Loader.modInit(Loader.java:263)
at cpw.mods.fml.common.Loader.initializeMods(Loader.java:604)
at cpw.mods.fml.client.FMLClientHandler.onLoadComplete(FMLClientHandler.java:203)
at net.minecraft.client.Minecraft.a(Minecraft.java:426)
at net.minecraft.client.Minecraft.run(Minecraft.java:735)
at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT 42a725de ----------
Also I am hosting a server and the command prompt says - "44 mods loaded
Optifine OptiFine_1.2.5_HD_S_C3
Minecraft Forge 3.3.7.135
FML v2.2.48.135
Forge Mod Loader version 2.2.48.135 for Minecraft 1.2.5
mod_CodeChickenCore : Initialized (modpack.jar)
mod_MinecraftForge : Initialized (modpack.jar)
mod_NotEnoughItems : Initialized (modpack.jar)
mod_ReiMinimap : Initialized ([1.2.5]ReiMinimap_v3.2_03.zip)
mod_IC2 : Initialized (industrialcraft-2-client_1.97.jar)
mod_IC2AdvancedMachines : Initialized (AdvancedMachines_4.0_client.zip)
mod_BuildCraftCore : Initialized (buildcraft-client-A-core-2.2.14.zip)
mod_BuildCraftBuilders : Initialized (buildcraft-client-B-builders-2.2.14.zip)
mod_BuildCraftEnergy : Initialized (buildcraft-client-B-energy-2.2.14.zip)
mod_BuildCraftFactory : Initialized (buildcraft-client-B-factory-2.2.14.zip)
mod_BuildCraftTransport : Initialized (buildcraft-client-B-transport-2.2.14.zip)
mod_AdditionalPipes : Initialized (buildcraft-client-DA-additionalpipes-2.1.3.zip)
mod_ComputerCraft : Initialized (ComputerCraft)
mod_CCTurtle : Initialized (ComputerCraft)
mod_CraftingTableIII : Initialized (CraftingTableIIIB1.8.zip)
mod_mocreatures : Initialized (DrZharks MoCreatures Mod v3.6.2.zip)
mod_EnderStorage : Initialized (EnderStorage-Client 1.1.1.zip)
mod_EnderStorage : Initialized (EnderStorage-Client 1.1.3.zip)
mod_IC2NuclearControl : Initialized (IC2NuclearControl_client_v1.1.9b.zip)
mod_ImmibisCore : Initialized (immibis-core_49.0.7_for_1.2.5-client.jar)
mod_ImmibisCore : Initialized (immibis-core_49.1.0_for_1.2.5-client.jar)
mod_IC2 : Pre-initialized (industrialcraft-2-client_1.95b.jar)
mod_InvTweaks : Pre-initialized (InvTweaks-1.41b-1.2.4.zip)
mod_MAtmos_forModLoader : Pre-initialized (MAtmos_packaged.zip)
mod_ModularForceFieldSystem : Pre-initialized (mffs_rev3_for_1.2.5-client.zip)
mod_IC2_ChargingBench : Pre-initialized (mod_chargingbench-client-1.95b-1.2.5-r1.zip)
mod_CompactSolars : Pre-initialized (mod_compactsolars-client-2.3.2.10.zip)
mod_IronChest : Pre-initialized (mod_ironchests-client-3.4.2.28.zip)
mod_RedPowerCore : Pre-initialized (RedPowerCore-2.0pr5b2.zip)
mod_NetherOres : Pre-initialized (NetherOres_Client_1.2.1.zip)
mod_PowerConverters : Pre-initialized (PowerConverters_Client_1.3.4.zip)
mod_Railcraft : Pre-initialized (Railcraft_Client_5.2.4.zip)
mod_RedPowerControl : Pre-initialized (RedPowerControl-2.0pr5b2.zip)
mod_RedPowerLighting : Pre-initialized (RedPowerLighting-2.0pr5b2.zip)
mod_RedPowerLogic : Pre-initialized (RedPowerLogic-2.0pr5b2.zip)
mod_RedPowerMachine : Pre-initialized (RedPowerMachine-2.0pr5b2.zip)
mod_RedPowerWiring : Pre-initialized (RedPowerWiring-2.0pr5b2.zip)
mod_RedPowerWorld : Pre-initialized (RedPowerWorld-2.0pr5b2.zip)
mod_TubeStuff : Pre-initialized (tubestuff_49.0.7_for_1.2.5-client.jar)
mod_TubeStuff : Pre-initialized (tubestuff_49.1.0_for_1.2.5-client.jar)
Balkon's WeaponMod : Pre-initialized (WeaponMod.zip)
mod_WirelessRedstoneCore : Pre-initialized (WR-CBE Core-Client 1.2.2.1.zip)
mod_WirelessRedstoneAddons : Pre-initialized (WR-CBE Addons-Client 1.2.2.2.zip)
mod_WirelessRedstoneRedPower : Pre-initialized (WR-CBE RedPower-Client 1.2.2.1.zip)
Minecraft has crashed!
----------------------
Minecraft has stopped running because it encountered a problem.
--- BEGIN ERROR REPORT 5f97ded2 --------
Generated 04/07/12 16:26
Minecraft: Minecraft 1.2.5
OS: Windows XP (x86) version 5.1
Java: 1.7.0_05, Oracle Corporation
VM: Java HotSpot™ Client VM (mixed mode), Oracle Corporation
LWJGL: 2.4.2
OpenGL: ATI Mobility Radeon X1400 x86/SSE2 version 2.0.6068 WinXP Release, ATI Technologies Inc.
java.lang.IllegalArgumentException: Slot 249 is already occupied by ic2.common.BlockTex@1ac1928 when adding ic2.common.BlockTex@1a7c53e
at pb.<init>(Block.java:251)
at pb.<init>(Block.java:284)
at ic2.platform.BlockCommon.<init>(BlockCommon.java:12)
at ic2.common.BlockTex.<init>(BlockTex.java:30)
at mod_IC2.load(mod_IC2.java:154)
at cpw.mods.fml.common.modloader.ModLoaderModContainer.init(ModLoaderModContainer.java:324)
at cpw.mods.fml.common.Loader.modInit(Loader.java:263)
at cpw.mods.fml.common.Loader.initializeMods(Loader.java:604)
at cpw.mods.fml.client.FMLClientHandler.onLoadComplete(FMLClientHandler.java:203)
at net.minecraft.client.Minecraft.a(Minecraft.java:426)
at net.minecraft.client.Minecraft.run(Minecraft.java:735)
at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT 42a725de ----------