Right clicking, with wheat seeds in hand, on a crop with something growing already still uses up seeds.
Happened in v1.95 also, upgraded to see if it was something that got fixed, but still happens in b.
This should work like fertilizer/water, so it won't accept more.
I only tested with wheat seeds, didn't try seed bags, or cane.
*Edit* Tested with Cane, and it worked as it should. Didn't use up any.