Latent images and microbolometers
The "double image" you described is most likely a latent image. This phenomenon is only found in microbolometer TICs. A hot image or source can imprint itself on the detector by warping the microscopic structure. After a few "shutters" it will clear.
It also could be a case where the TIC performed a "shutter" or non-uniformity correction (NUC) without the shutter in place. This usually manifests itself as a ghost image that looks like a negative image (dark where it should be light). This isn't what you described, so I suspect it isn't a bad NUC. Once the shutter fires correctly during NUC, this phenomenon will clear as well.
Non-microbolometer TICs don't experience this because they have a chopper that corrects the image 30 times a second.
I hope this helps.
ISG Thermal Systems USA, Inc.