Comment 6 for bug 883459

The problem occurs only on GMail's POP3 account (actually that's the only account that uses filters).

Two more observations:
* only filtered emails get duplicated/resurrected (e.g., only when emails were moved from one folder to another);
* this only happens when the filter has to process several emails at once (I do not remember seeing this behavior when only one email comes - only when there are multiple emails).