My female friends always slap each other's boobs, is it normal for teenage girls?

I have a group of female friends whom I am very close to and I often see them slapping each other's boobs to the point where they would challenge each other to a "boobs slapping challenge" to see who can withstand more hits to the boobs. I've seen this happen not once but many times. The question is, I'm really curious if this is a normal behaviour among teenage girls?
My female friends always slap each other's boobs, is it normal for teenage girls?
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