You can make unlimited comments on your own gallery images, but only one on other people's (previous comments will disappear in lieu of the newest one)....I think it's always been that way. (A double-posted comment would be different, that's a "glitch", rather than 2 separate comments - like when you inadvertantly double-post in a forum cos it's taken so long you think it hasn't worked, and repost, and then go back and delete the extra post.)