why same key can't be inserted in map [duplicate]

I searched everywhere and results tells everywhere is like `same key can't be inserted in map

I am asking why ? why we can not insert it in map. i want to know the reason not the rule like it can't be inserted.

i mean when we call insert() in map , for same key it will return false . But my doubt is why false, as its internal DS is tree what make the tree to fail the insertion with same key