It's very common, actually. Typically it means you have used both forms of the url in incoming links to the domain.
There's a school of thought that says that there is a risk that google will decide the two urls are duplicates (which typically they are) and will devalue the serp ranking of one of them. The risk is they will devalue the one you want to keep high in the serps.
So, the usual advice is to pick one form of the url and always use it in all linking.
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