I don't know of there is any module to do that, but if you want to do it manually, you could have a hash in which the keys will be the tag name (probably stripped of the < and >). Parse your file, for each opening tag, increment the value of the hash for that tag. For each closing tag, strip the </ and >, see if the hash exists for that tag and if the value is 0: if the hash does not exist for that tag or the value is zero, add an opening tag; if the value exists and if the value is larger than 0, decrement the hash value for that tag.
EDIT: well it seems there is a module, I had not seen the previous answers when I started to post mine. Forget the above and use the module.
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