Answer: Syringa microphylla Superba, a little leaf lilac, is described as blooming in the spring and "often reblooming in September." It is rated zone 4 to 8, so it should do fine in central NYS. The plant is available from Forestfarm (www.forestfarm.com) and Caroll Gardens, ph# 800-638-6334.
Another lilac variety, Syringa tribida 'Josee' is a relatively small shrub--4 to 5' tall--that blooms three or four times a year. It is available from The Great Plant Company (http://www.greatplants.com/). I didn't see it listed in their online catalog, but it is in their print catalog.
I can't personally vouch for the reblooming aspect, since I haven't tried them.... yet!
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