Probably not very good info, since this is ex post facto, but just logging this.
Yesterday I posted an observation. This morning, another user flagged to me that the observation was a duplicate, and indeed when I looked at my observations it certainly appeared to be so.
Easy enough, I deleted one of the observations. They both disappeared.
That tells me that it’s unlikely I duplicated the observation, but in fact iNat had; clearly somewhere deep within iNat these two observations were linked.
I’ve been on both ends of this. In Explore (and in Identify, but less often so), iNaturalist will occasionally show the same observation thumbnail multiple times, sometimes right next to each other, other times spread apart. If you click on it and look at the URL, it’s the same.
I’ve seen the “duplicate” observation problem frequently with the Safari browser on a Mac for various iNat pages (Observations, Explore, etc.). I’ve recognized this as a browser issue, not one within iNaturalist. As @natev suggests, always check the URLs before deleting any supposed “duplicate” observation.
More detail: When I view a set of observations on iNat in a Safari browser window (about 12 or 16 are displayed depending on the size of thumbnails), then scroll down to look at the next set, sometimes the browser cannot properly keep track of which thumbnails were displayed and resets to an earlier point in the list, thus throwing up what appear to be duplicate observations. Actually the browser has just “back-tracked” to an earlier point in the list and is re-displaying some thumbnails that have already scrolled by. I switched to Chrome (Mac) and have not seen the problem repeated.
Ah, good point! In this case, it was two different URLs.
I don’t have a solution but I can confirm this has happened to me before. I think updating the app fixed this a while ago and it hasn’t happened since.