finding and counting substring appeared in a string how please?

Hello everyone, I have sample line of string:

This is a test. <br /> This is a test on text. <br /><br /> This is a test on text. <br /><br /><br /> This is a test on text. <br /><br /><br /><br />

I want to find <br />. If it happens that <br /> appears more than 2 times next to each other, replace it with <br /><br />. How cound I count and check that? The result I want is something like this:

This is a test. <br /> This is a test on text. <br /><br /> This is a test on text. <br /><br /> This is a test on text. <br /><br />

Thank you very much. 
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