Testing three times?

Hi ladies, I have what might be a silly question. I was diagnosed with gestational diabetes around 16 weeks. Everything has been going great since then. Blood sugar has only been high a couple of times in the beginning. I'm now 21 weeks and my doctor says she wants to do the tolerance test again. I asked her why and she said "because we tested you early" and walked out of the the room. I don't see the point in testing me again when we know I have it. Is it normal for them to test again?