When updating from 2.0.12 to 2.0.14 the mod downloaded the new jar file "ForgeMultipart-universal-1.6.4-1.0.0.219.jar" as required, but during game start I was informed that the same mod exists in two versions. The other "mod" in question was the previous version of that file above.

Expected: Any old library version should be deleted during download.
Work-around: I manually deleted the old ForgeMultipart jar.

Specifically a bug in old versions of Deploader and FMP

Works like expected to, not sure why that happened.