My boyfriend has a female friend that I find suspicious. Should I be concerned?

My boyfriend has a very specific type and she fits it to a tee. He is also very attentive to her (she was calmly standing in line at a food truck and he went to check on her to see if she was okay). He also seems to value this friendship a lot for them to have met four months ago.

When I talked to my friends about it, half told me not to worry about it and others have said that this is his back-up girlfriend (basically in the event that we break up, she will be the next woman that he dates).

Am I overthinking things?
My boyfriend has a female friend that I find suspicious. Should I be concerned?
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