Yes, there is more than one Invisibility Cloak, but there are other things about Harrys that make him suspicious that it's the Hallow - the fact that Dumbledore was so interested in it, it has repelled spells in the past and no matter how tall the three get, it seems to always cover them (apart from odd moments with their feet, but that never appears to be a problem).

Of course, it may not have been an invisibility cloak - there are all manner of spells to make one 'unseen'... Dumbledore refers to them when he says he has no need of a cloak to become invisible.

LTS