When identical frequencies that exist in two different layers (channels) are combined, you invariably get a gain boost at those frequencies.
If you take two sine waves of the same frequency and amplitude, and sum them you will get a gain increase of 6dB (example below).
The waveform on the top is at ‑9 dB, and when duplicated and summed into a new single mono file we get a value of ‑3 dB. This is important information to take on board and nail into your brain: you can imagine what happens when you have a mix with a huge number of channels all summing and clipping the output because shared frequencies are always summed at the output.
To fully appreciate and understand how summing works please watch the video:
A good way around this problem is to leave ample headroom in each source channel (input), so that when two or more files (channels) are summed they do not exceed the ceiling and clip the master outs.