Ahhh I'll try not to say anything too subjective, since I am a huuuge fan of Coldplay... hm.
Not gonna talk about Talk & Computer Love, as it was said, that was openly admitted to be inspired by it.
Concerning Viva La Vida, there are thousands of songs out there that sound pretty similar to others. Maybe they stole it. Maybe they didn't.
I can't count how many times it has happened to me that I wrote something, let it be a story, a poem or song, and later found out that it sounded pretty much like something else. Sometimes something I loved and got inspired by without realising it, but sometimes it wasn't even something I knew, and only found out about it afterwards.
What I'm trying to say this, things like that happen. To anyone. And Viva La Vida is still one of Coldplay's best songs, imo, and although it might sound a tiny bit like that other song there, it's still completely different. It's still "Coldplay".
Anyhow, even if they stole, I'll admit it sucks - but that's not gonna stop me from loving them and their music.