I get spit back on dual coils after filling... If your build does this then instead of filling from the top unscrew the Base and fill from the bottom. Other alternative is to fill from top, turn cap on one turn, turn over and tighten
If you still get spit back then press fire button while...
Check that the coil is properly fastened too and that you don't have hotspots.
I trim my wick pretty short so it justs gets held by the clamping ring.
Separate issue I'm finding is if I use bigger coils I get a lot of spit back, I crank it up to like 50w and it goes away eventually but until...
Mine spits a bit so I'm guessing the wick is really saturated after filling, no real leaking though... Use more cotton maybe. I find you can use a lot of you cut it short from the bottom as per kalashnikovs pic.