Dobie Gillis is set in Central City, a fictitious city in the Midwestern United States (the original short stories are explicitly set in the Minneapolis–Saint Paul area).
Central City, the same setting for the 1990s televersion of ‘The Flash’, seems like it was a gentle small town of the early 1960s. So it doesn’t seem likely that it could be possible that at least five young ladies, perhaps even six, could be so identical in appearance.
From the IMDb:
The Many Loves of Dobie Gillis
- Pilot (1959) ... Girl #1 (uncredited)
- The Flying Millicans (1960) ... Aphrodite Millican
- Dobie's Navy Blues (1960) ... Myrna Lomax
- Like Mother, Like Daughter, Like Wow (1961) ... Hazel Grimes
- The Sweet Success of Smell (1962) ... Elspeth Hummaker
- Flow Gently, Sweet Money (1962) ... Linda Sue Faversham
However, I think a likely candidate for the father of all of those “identical cousins” could be Professor Millican. That epitome of physical fitness and acrobatic prowess – good for climbing out of bedroom windows – might have been quite a Lothario among lonely wives of Central City.
And carrying that idea further, it could be the Millicans visited other locations in Toobworld around the late 1930s, early 1940s and foisted more of these identical cousins on their citizenry. (Especially with shows like ‘Sam Benedict’ and especially ’77 Sunset Strip’.)
And who knows? Maybe even to Gotham City as well....
But... but... you're my sister!