Ok, so this will work on all versions of minecraft (everything from 1.4 to 1.11.2 and beyond) and is my preferred method and is so easy, a noob can do it without breaking a sweat (in most cases).

 

Here's a scenario:

 

ok so you just installed a mod and minecraft starts crashing upon the mojang splash screen (or in some cases, the black screen before the mojang splash screen even appears).

 

now this is especially common when you have over 85 mods installed (mine is 89 so far ) and any one of the currently installed mods can interfere with the new mod.

 

in this case, i'm trying to use the floorball craft mod.

 

in the crash report you see:

"java.lang.IllegalArgumentException: ID is already registered: 117"

 

this is where you might need to take note of the ID number (only if you have troubles remembering stuff)

take a pen and paper (or your preferred word processor) and note the ID (which i have highlighted the position of in red and bold)...

 

ok so what you should do at this point is delete the new mod (not entirely as i doubt you want to redownload the mod) or put it in a backup folder.

 

ok so i'll use my current username for this example (MrThrall)

Start Minecraft and load a world (doesn't have to be your main world, you can create a new world if necessary), and input the command "/give MrThrall 117 1"

 

 

At this point, the item you just recieved can be from one of 3 possible categories.

 

 

If the item is from a mod:

 

 

take note of what mod the item is from and close minecraft.

then what you have to do is either

 

 

a) delete the mod the item came from

 

 

or if you are really advanced (a.k.a capable of developing your own mod from scratch)

 

 

b) actually modify the mod itself and reassign the conflicting item a new ID.

 

 

 

 

If the Item is a Vanilla Minecraft item (worst case scenario):

 

Contact the mod developer and notify them of the conflict (a copy of the crash report may also be helpful) as chances are, if this is your current problem, this mod was probably rush-ported to a new version of Minecraft and they didn't bug test.

 

 

 

OR you could use anything past 1.5~ and let forge take care of it all automatically for you.