I've noticed a strange bug where if all slots are full, but one still have an incomplete stack, the EMC condenser stops duplicating items.
Example:
All slots are full, one slot has 7 items
Result: EMC Condenser stops duplicating items and never fills up the stack.
This is especially true and noticable if you take items out of the condenser with hoppers or similar methods at the same time as it's duplicating items.