You'd think that being blown up & resurrecting yourself would be distinct enough origins that lawsuits (or threats thereof) would follow when another character has essentially the same origin, but no, at least three heroes of the 1960s had that as their origin.

Captain Atom - Space Adventures #33 March 1960 - Charlton
Doctor Solar - Doctor Solar #1 October 1962 - Gold Key
Nukla - Nukla #1 October 1965 - Dell

I suppose that since Captain Atom had stopped appearing in October 1961 is why there was no lawsuit when Gold Key created Doctor Solar. Although, who knows whether the people at Gold Key even knew of Captain Atom? (Reading behind the scenes stories about Dell/Gold Key I get the general feeling TPTB didn't pay much attention to the rest of the comics world.)

Does anyone know of any earlier characters who had as their origins being blown up & coming back together?

As for later ones...

Wildfire in the Legion of Super-Heroes, although he didn't so much come back together as needed a containment suit to inhabit.

I'm not sure if Firestorm, The Nuclear Man & his villain Multiplex were actually blown up or just exposed to radioactivity.

Doc Manhattan of Watchmen doesn't count because he was supposed to be Captain Atom, but the publisher made the writer change him.

Marv Wolfman's Great F**k Up Of 1985 threw a lot of origins out the window.