An idea is worth nothing without execution. I found that it doesn't matter if you share your idea with friends/family/colleagues or don't. 99% of people won't execute on them :)
It's commonplace to have similar ideas with competitors. The thing is to find your uniqueness. Having a novel design is the key to success. Think before your competitors and the market.
It's a valid concern for sure. I think it depends on how unique & defensible your idea is. If it's super niche or has tech that's hard to replicate quickly, prob less to worry about. But if it's more of a generic concept, moving fast is key to gain traction before copycats emerge. Having a strong brand/community also helps. And ofc, execution is everything - an okay idea executed amazingly will beat an amazing idea executed poorly. Just my 2 cents!