I agree with everyone else -- you should disclose, and I see no problem with this. Let me raise a subtler issue:
A few years ago, I had the experience of being repeatedly sent a paper where the result was correct but, in my opinion, not close to significant enough to appear in the journals where the authors were sending it. I sent a report to this effect twice but, when I was asked to referee at a third journal, I declined on the grounds that the authors deserved a second opinion. If the paper was actually wrong, I would have kept saying so but, in a subjective case like this, I thought it would be wrong to be the referee who killed it single-handed.