Swap buffers twice does not show the first scene
If you swap buffers, you have a triangle in your screen and a empty color buffer in the other side.


No, you do not. Buffer swapping does not necessarily mean that the current contents of the front buffer are preserved. When you do a swap, the contents of the back buffer become the front buffer, but the state of the back buffer is undefined after the swap.

Some implementations use a true swap, where the two buffers really are switched. Other implementations copy the data from the back buffer on a swap, leaving the back buffer as whatever was there before. Still others could do something else.