Ep145 Deeper Dive- Was Dovid really married to sisters, and how could Shaul give them to other men?

The Gemara in Sanhedrin 19b discusses (1) What was the problem with Dovid's marriage to Meirav and Michal? (2) How could Dovid have married sisters when this is prohibited? (3) If Dovid was married to both of them, how could Shaul have given them to other men?