It certainly looks like a double, the kind where all the massing is contained w/in confines of the cup (plus a bit of more ruffling on rim). I traced the ancestry back a few generations just to see if any doubles (unlikely indeed) might have been used, there was no evidence of said double in gene lines. I checked for small white bell doubles and couldn't find any. Sport may be the only logical explanation. If it is, you might consider contacting some breeders or professional growers to multiply stock. There have been several instances of sports spotted, named, multiplied, and sold (Thalia/Thalia Sun). The originator of Petrel, Grant .E. Mitsch is still around (I think) and active in the field; I've seen him post on the DafffNet site before (I think). He used to have his own web site, but the site seems absent now. Brent Heath of Brent and Becky's know him.
At any rate, it's a cool looking little flower and sets well where it's situated in your white 'colony'.
