You indicated your newest snake had mites...most likely a couple of those mites left the snake and found a nice place in your home to lay eggs. Once these hatch, you have more mites to deal with.
Also, if you have native snakes in your area, it is also possible to pick up mites on your shoes and clothing, especially if you take short cuts through fields and other natural 'snake' habitat. As mites are not easy to see, very easy for them to completely escape notice till they are all over your snakes.
As you indicated the mites keep reoccurring all the time...it may be necessary to do a complete home spray..to kill all mites and their eggs hidden all over the house.
I would start with a small room and spray it down...after a day or two to give some time for fumes to dissipate, move all your reptiles into that room. Then spray the reptile room and the rooms/halls leading away from it.
To help prevent an excessive building up fumes which can prove toxic to animals, spray only one room at a time, giving it a day or two to air out before doing the next room.
As Provent a Mite is expensive, your best bet is to get the human lice spray and use that to do the overall spraying.
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PHLdyPayne