I can only find three azaleas named Sweetheart. The newest one is 'Autumn Sweetheart', a patented Encore Azalea. It isn't double and is a re-bloomer.

The older ones are Pericat Hybrids.

One named 'Sweetheart Supreme' fits your description. It is semi-double, hose-in-hose, blush-pink, frilled, with a deeper blotch. Blooms late-midseason, medium height, dense, hardy to zone 8a, root-rot resistant. It was named by Robert Craig and Company.

The other Pericat Hybrid is 'Sweetheart', but it is a strong red named by Perkins-de Wilde and blooms early-midseason.

The Pericat Hybrids were developed by Alphonse Pericat, a florist in Collingdale, Pennsylvania. His plants are probably Belgian Indian Hybrids crossed with Kurume Hybrids. He introduced them in 1931 at the Philadelphia Flower Show. He distributed a lot of unnamed seedlings that were named by other nurserymen.