How to make element loopable with alpha channel in After Effects

1. Go to 1 second on the selected the layer, shift+cmd+D split the layer, and move the top layer to the end. 

2. Animate the top layer opacity from 0-100 (if your layer does not contain alpha, then you are done)

3. Animate the bottom layer opacity from 100-0

4. Change both layer's blending mode to Alpha add