are Hungarian "long" vowels kinda like double vowels in Finnish?
Kinda sorta yes, but not quite.
For o-ó, ö-ő, u-ú, ü-ű, i-í, the only difference in pronunciation is length, and they're pronounced roughly as their Finnish equivalents.
For a-á and e-é, not only length changes, but also the sound.
The a is like British English short o.
The á is like Standard Dutch aa, or Standard Italian, Standard French and Standard Spanish a.
The e is like British English, American English, Standard Dutch ans Standard German short e, or like Standard French è.
The é is like Scottish English ay, Standard French é, Japanese e, or Standard German and Standard Dutch ee.