handling geographic locations dynamically

I've built a website for vacation rentals and housing for rent. I've created a page where users can add locations themselves (up to 6 levels deep) but I have come across a problem. A user may add a location ie:

North America -> United States -> California -> San Francisco

Another user may arrive and add an area under SF which he assumes to be under SF ( say Silver Terrace ). Yet another user could add that same area but he would not look under SF and would add to be a brother node of SF

North America -> United States -> California -> Silver Terrace

So now we would have:

North America -> United States -> California -> San Francisco -> Silver Terrace

North America -> United States -> California -> Silver Terrace

which is of course a duplicate =/ I could tell the user adding Silver Terrace the second time that it already exists but the first user may have placed it incorrectly which causes a problem.

Even if I were to not allow users to add locations and I would do it myself or limit it to a few users, it would still be very difficult to correctly add all locations.

In other countries there also comes the question of including or not including the state/district/region and so on to be part of the path for locations...

I've read quite a few posts about this but I can't wrap my head around a good solution.

It is difficult to eliminate locations altogether because I use them for subdomains ie. san-francisco.example.com or california.example.com . Locations also allow users to browse 开发者_如何学Cby country/state/city/area/neighborhood/etc if a user doesn't want to (or can't) do so using a map.

I don't think there is a perfect answer for this but if you've got some suggestions, please let me know. Thank you so much.

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Another approach you could use, to automatically add sub domains with little user input, is a statistical approach. (This approach assumes you have no information about locations initially)

As a motivating example, let's say that you have 20,000 users on your system and 10,000 of these users put in a location (in the form of x -> (y ->)* z) that starts with "United States -> California"

Because "United States -> California" is an element which is found in such a large number of locations, you could be confident that "United States -> California" is, in fact, a valid location. This approach makes sense when adding subdomains because you may not want to add a sub domain until a significant portion of your population would become members of that sub domain.

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Have a hidden value attatched to the X->Y->Z path which denotes Z's ZIP code. That might be the easiest way to determine which paths are duplicates.