Only one vote is allowed per IP address per entry.
If it says 'You have already voted' then maybe someone was/is using the same network IP address and they have already voted. Normally all computers on a WiFi or wired network share the same router IP address (e.g. home, work, school, building, office...)
If you have access to a computer at another location (e.g. work, school, university, cafe...) you may use the IP address there to try and place a vote.
Normally competitions use cookies to determine if someone has voted. In the past If we used cookies for voting people cheat by deleting cookies and voting again. Although not perfect using an IP address is the least abused.
It is important to note that facebook does not allow its own competitions based on public votes. This is because it is easy for people to cheat with votes. Facebook also warns other companies that running fun public vote competitions with prizes attracts many problems with cheating and verifying true votes
At the end of the day the part of the competition 'by public vote' is basically who has the most friends. Some would say that is unfair and it should be based on how good the image is...
That is why we also judge the photos independently for additional prizes and list those as winners 'as picked by hop'.