Our server uses custom made tools such as a tier of copper and steel. However, it seems that under certain conditions that dying with some of these tools causes them to duplicate on death.

This can be reproduced mainly by dying to a creeper/creeper explosion. The custom tool will duplicate itself on death and drop. I haven;'t been able to verify it myself, but others are reporting to me that dying to any hostile mob will do it sometimes, but it happens most often to them with creepers/explosions. 

I can verify this for sure happens with custom swords and pickaxes, haven't yet been able to thoroughly test with the other tools but I'd expect the same results. The duped items share the same durability stats as whichever is the original.