I hear people say something like this:
How did you come to know that I bought apples?
Compare with:
How did you know that I bought apples?
To me, they both seem to mean the same thing. Does the 'come to' change the meaning or is it redundant? That is, is 'coming to know' the same as 'knowing'?
Both questions are asking how an individual acquired knowledge about the apples and can be used interchangeably in most circumstances. There is however a slight difference in nuance.
How did you come to know? emphasises questioning the mechanism by which the information was obtained. Did you search for the information, stumble across it or were given it? It also carries an implicit question of why did you want to know?
How do you know? still questions the mechanism and, as you say, carries the same basic meaning, but has no implicit why.
