I think it depends. I think in many companies it's unusual, but I've also worked for a company where it was not unusual for someone to leave, work somewhere els

I think it depends. I think in many companies it's unusual, but I've also worked for a company where it was not unusual for someone to leave, work somewhere else for a year or more, and then come back. But I think it was the only company where I've seen that happen. I think it depends on why people leave. People who are fired because of the way they function are unlikely to get rehired, but people who are fired because the company is in financial trouble, might get rehired when the company is in calmer waters. Similarly, someone who leaves because they don't like it, are underpaid, hate the work environment, their manager, etc, are unlikely to return. But someone who likes it, but wants to try something else, might want to return. The main risk that I see is that a company that knows you might see you as the person they used to know, and ignore any growth you may have had in the mean time. Rejoining a former employer requires open minds on both sides.