FFmpeg Single Input, Seek to Multiple Outputs
There are two basic ways to do this:

1) feed file as multiple inputs and extract one image from each input

ffmpeg -ss 280 -i sample.mp4 -ss 300 -i sample.mp4 -ss 302 -i sample.mp4
-map 0 -aspect 1.7777777777778 -vframes 1 -qmax 9 -qmin 8 -bt 10000000 -y -pix_fmt yuv420p out1.jpg
-map 1 -aspect 1.7777777777778 -vframes 1 -qmax 9 -qmin 8 -bt 10000000 -y -pix_fmt yuv420p out2.jpg
-map 2 -aspect 1.7777777777778 -vframes 1 -qmax 9 -qmin 8 -bt 10000000 -y -pix_fmt yuv420p out3.jpg


This method can get unwieldy with many targets or if the input is read from over a network.

2) feed it as one input but only decode keyframes and output as one stream.

ffmpeg -skip_frame nokey -i sample.mp4 -vf "select='lt(abs(t-280),0.2)+lt(abs(t-300),0.2)+lt(abs(t-302),0.2)'" -aspect 1.7777777777778 -qmax 9 -qmin 8 -bt 10000000 -y -pix_fmt yuv420p -vsync 0 out%d.jpg


This allows a leeway of 0.2 seconds in identified keyframe timings.