When someone uses two question marks, it is usually to emphasize something in response to what was said before. This is often done to express a strong emotion or to convey shock. The double question marks are usually stacked on top of each other, and the meaning changes with each pile. For instance, if someone inquires "What time do you want to meet?", it could be interpreted as a simple, modest, and friendly inquiry.
It conveys the point of view, elicits a response, and seeks a solution. However, if the same question is asked with five question marks ("What time do you want to meet???"), it could be interpreted as playful or sarcastic. In this case, the extra question marks are unnecessary and could be seen as a typo. If you were speaking to the person on the phone, you could even sing the whole question with five question marks. This could be seen as an indication of the speaker's enthusiasm or excitement.