Louisiana and Washington have had top two but most, if not all, of their elections have had one person from each party finish in the top two in the primary.
Get ready for the Wild West in California. Because they'll likely be in the same district, Howard Berman and Brad Sherman will face off in June and then again in November. I'd guess that they'd call off the dogs in June, as each should coast into top 2. But you'll have the unusual circumstance of two high profile Democrats going at each other, in a possibly ugly race, in November. That could siphon donations from candidates running against Republicans and dominate media time.
Parties usually cringe at a knock down drag out primary, but it's part of the landscape. Here they'll be having that in November, in a race they can't lose. I imagine that'll give Steve Israel headaches.