What they are talking about is if you take another dose of the same drug within (11 — Addiction Rating) Weeks.
In the case of Psyche that would be 11-6=5 weeks.
Edit. Never mind, you are talking about overdose. I am talking about addiction
My question is why would you already take the same substance twice?
You typically don't willing take the same drug twice. Having said that, a good reason might be that the drug is about to wear off and you wish to extend the duration rather than letting it wear off, take the drug again, and then wait for the new dose to kick in. There are also drugs where you will crash when the effect wears off. In some situations it might be better to stay high and risk the overdose and take the crash at a later time (you typically don't want to crash during a combat scene, for example).
Overdose rules also applies when you take another drug that share a similar bonus. If you are currently on Cram you get +1 Reaction and +1D6 Initiative Dice. Say you run into a social situation and you, while cram is still active, inhale or inject yourself with Novacoke (for that higher Charisma and Social Limit) you risk overdose (because it, just like Cram, also share the +1 Reaction property).
(for future reference, rule questions that are not SR6 normally go here:
https://forums.shadowruntabletop.com/index.php?board=29.0)