Hey Smitty, i've come across quite a few over the years as well, 2 noteable finds were a 2-headed bicentennial half dollar and a 2-headed 1996 half dollar, the only way i found them was because they made a thud sound rather than a clink sound, I usually open them and toss them into a jar for anything unusual, i just can't put a fake/foreign coin into circulation.
nice find,
hh
buff