"You have already voted..."

Almost every time I try to give cred, it says that I have "already voted for this bimbo".
I share a computer and internet connection with my sisters, could they have beaten me to it?
I do find it unfair though.
If I was at my mums I'd have no problem. I have tried clearing the cache, to no avail.
Why shouldn't I be able to give cred to the Bimbo's I believe are best?