Secure MAC implies that probability of same tags on different messages is negligible So let any secure MAC (message authentication code) be given.

Intuitively, I think it is clear that the probability of getting the same tag on two different messages is very small, i.e. negligible. I want to prove this statement mathematically.

How can we 


formulate this in an exact manner (because, negligible in what? I guess negligible in $|k|$, where $k$ is the chosen key - I am not sure about this)
(once we have a precise formulation) prove this statement?


Supposing that this probability is not negligible, we'll have to construct a PPT adversary $\mathcal A$ that, somehow, is able to "beat" the system (which would be in contradiction with the fact that the MAC is secure).
Is this a homework problem?

Here's a hint: Suppose that the probability of getting the same tag on two different messages is $p$.  Show how to construct an adversary that breaks the MAC (i.e., forges a valid tag), and that has success probability $p$.  Assuming the MAC is secure, what can you conclude about the possible range of values for $p$?