I was watching episode 4×02 when I saw that:

Sorry the image was broken, I leave you the source link: https://imageshack.com/i/nrffvap and https://imageshack.com/i/nmi73fp

I was watching episode 4×01 when at the end of the espisode, PETER CAPALDI APPEARED! I was like “oh my god what is he doing there ?!”  So there are two doctors at the same time in the same world… Interesting.