Inserts into a Federated table with a auto increment column causes the insertion of 0's into the auto column and if you have sqlmode set to 'no autoincrement on zero' you get duplicate insert errors. Even if specifically pass a null value to the auto column this occurs.
I think the engine is setting defaults for any missing/invalid columns before passing to host and autoinc columns are not null so defualt is 0.
I have worked around it by removing the sqlmode (it was a legacy setting)
I have been using Mysql for some time and I am now trying MariaDB (I hade hoped this bug was fixed!)