there's no way it can harm your baby so use it as often as you need to hun for the reassurance
xx
I'm sorry to say this and I don't want to start a row, but this is simply not true.
Dopplers are more constant sound waves than a standard ultrasound, and there has been no research done to prove their safety, used for prolonged times or this often. Some studies have found evidence of cavitation, which is where the spaces in the tissues (present in all mammals) which are filled with gas are affected by the sound waves and implode, causing extreme heating. A couple of degrees can really affect a developing baby, which is why we are warned not to overheat, use really hot spas etc. There have also been studies showing the soundwaves being reflected off the internal curve of the skull, thereby magnifying and concentrating the waves many times greater in the brain area of the developing baby.
Have a wee look at this, written by a midwife and backed up by studies, all referenced (I can't remember all the right references), explained way better than I can: https://www.midwiferytoday.com/articles/ultrasound.asp
https://www.motherandchildhealth.com/Prenatal/sarah/ultrasound.html scroll down to "the biological effects of ultrasound" to get a better explanation of cavitation than I gave!
Everything relating to ultrasound also applies to dopplers, as I say dopplers are stronger as they use a constant sound wave, where ultrasound is generally intermittent waves. It's up to you if you still want to use it or not, but is sounds like you would appreciate some actual facts to help you decide. HTH.