Was there any reason for having two characters named Danziger? Nothing wrong with the name but not exactly common like Smith or Jones etc.

Nope ... nothing mystical ... just studio writers recycling studio material.

Not so strange when you consider the fact that the unmistakable monikers of "Victor Norris" and "Tanya Baker" were used (together again) in an early season episode of "Law & Order" (the best incarnation of the entire series).

Who's the second Danziger?

I don't know what order you mean but it was Hayden Danziger in Troubled Waters, then Jason Danziger in Bye Bye Sky High....
I wondered if Lev or Link had a friend called Danziger, or a pet dog or parrot etc.

What about the two Brandts from Old Fashioned Murder and Bye Bye Sky High IQ? Always wondered why they used the same name 2 episodes in a row.

Am I the only one that likes to think that the two Danzingers are related to create a cohesive universe?

Maybe a close examination of the films and their dialogue would reveal something, some connection?

Sometimes names are recycled because they're names that are already pre-cleared as not having any real life counterparts that could get them in legal trouble. A couple other "Columbo" names used before "Columbo" include "Jerry Parks" (Requiem For A Falling Star) in a 1968 "Hawaii Five-O" episode while "Grace Willis" (the married name of Janet Leigh in "Forgotten Lady") is in another 1968 "Five-O" episode.

Of course sometimes names that were used on TV could never be used again years later like the 1957 "Perry Mason" episode where the killer is "Willard Scott" and there is also a 1973 "Rookies" episode with a one-time criminal named "Jimmy Carter" (complete with a line "I'm not in trouble, Jimmy Carter is!" that is even more hilarious with hindsight)

Yeah I agree with the names that are already pre-cleared premise. Keep in mind these episodes were written pre-Internet, so it was harder to research new names. Why not recycle one that you know isn't going to get you in trouble.