If I already have a chin implant, is it safe to get lipo?

 If you are having your double chin treated with liposuction, it should not interfere with the placement of your chin implant. Just let your board-certified dermatologist know about the implant so that your procedure can be planned around it.  

Yes if you have had previous chin implant it is safe to have chin lipo later and vice versa.  Furthermore, this two procedures are actually often done simultaneously.  The chin implant is placed at a deeper layer than the chin lipo which removes skin fat.  The chin implant sits over the bone.  

You can have liposuction.  You might also consider Kybella as a possible option for your double chin.

Liposuction in the submental area under the chin should not be affected by prior placement of a chin implant. The implant is typically placed along the lower border of the jaw, and the small incision that is created for liposuction is typically well below/behind this. There are several issues that can cause the appearance of a double chin, and liposuction is only one treatment alternative. I would recommend seeing a plastic surgeon who specializes in facial aesthetics to evaluate whether this would be the best option for you.
Best wishes,

It would be fine to have liposuction under the chin with a chin implant in place.  The puncture for the liposuction is normally made just underneath the chin and below the chin implant.