Can I say どうぞよろしく if I have met this person before?
Or is it like はじめまして where I can only use it once with someone? Thank you!

Question

Answer

you can use it every time but hajimemashite is just for first time.

good luck!

Answer

はじめまして！
'はじめまして' literally means 'for the first time', so the usage is limited only to the situation you meet someone else for the first time.

If you say this phrase to the persons whom you met before, they may be offended from their perspectives cuz it is considered as rude.