To expand a bit on what TripeHound said in his comment, past performance is not indicative of future performance, which is why the best advice is to ignore if you already own the stock or not.

If the stock goes down, but you've done your research and think it will come back, then investing more isn't a bad idea. If the stock is doing well and it will continue to do well, then invest more. Treat investing more into a stock you already own as a new investment and do your research.

TL;DR of your question, it's a very case-by-case basis

To expand a bit on what TripeHound said in the comment section, past performance is not indicative of future performance, which is why the best advice is to ignore if you already own the stock or not.

If the stock goes down, but you've done your research and think it will come back, then investing more isn't a bad idea. If the stock is doing well and it will continue to do well, then invest more. Treat investing more into a stock you already own as a new investment and do your research.

TL;DR of your question, it's a very case-by-case basis