Barry, just installed 3.0.14 (but also noticed this in 3.0.12) but as Ive been away from my PC i hadnt gotten around to letting you know. In the Decades screen freebox is allowing you to click on the same song via the 'add song to queue' button and load up duplicates in the queue.
I tried adding the same songs via the album screens but no duplicates were allowed there.
You are correct i will fix this now.