You and your citing.
I'm going to cite kreynolds official D&D Rules forum posting on this one
"One check per source."
And from the SRD
"To cast a spell, the character must concentrate. If something interrupts the character's concentration while the character is casting, the character must make a Concentration check or lose the spell. "
So if something interrupts then make a concentration check... if something else interrupts make a concentration check. It doesn't specifically say more than once but it does say what i just paraphrased, quoted and quoted someone saying.
Your humble sarcastic troll
DD
edit: maybe i should read before i post