Your friend on T-Mobile has an iPhone 6 and you're getting duplicate messages and also if she sends a longer text it's sending them as 1 of 3, 2 of 3, 3 of 3? Is that what you mean?
What phone are you using? Has this always happened or just started? Is it for every single message she sends?
Most likely it's something that will work it's self out. It could be she has a low signal and so her phone keeps trying to send messages, in turn sending it multiple times.
This happens occasionally to me, Verizon to Verizon, sometimes it lasts mere hours, and sometimes 24+ hours, but not to every single message, but always from the same individual who is experiencing service issues on their end.