Haha yeah, I meant 'looper' but auto-correct didn't like that. Thanks, that makes it pretty clear, then. I'll just go back to using the mini kp post-mixer, pre-speakers.

By popper do you mean looper?

The Behringer send/receive is post-fade, which means it's only useful for effects with "add" something to the sound - e.g. delays, reverbs, etc. Anything which modifies / cuts into the sound itself (e.g. filters, loopers, eq etc) won't work - you'll get the "doubling" of FX and clean you describe.

Have you turned the FX depth down somehow?  That'd be my first guess.
Also, looks like the eurorack ub802 has a mono send/stereo return.  Do you have a mono quarter inch (TS) to RCA going out to the mini KP?  Because, if you are using a stereo quarter inch (TRS), only the left channel will be sent to the mini KP, and the effects will be coming back to the board in stereo and can make it sound funky.

I've tried using a mini kp though a Behringer Eurorack ub802's fx send/stereo aux return and for some reason it's like the effects are only at a mix of 50%. I know that would suggest a stereo or connector problem but I've made sure they were the right stereo/mono cables and connections.

I was wondering if anyone else has had a problem like this. Whether with this equipment or not.

The problem becomes most audible when using the kp's Popper functions and the audio still plays both fx'd and clean.