That's a valid concern. It doesn't happen much, but it's still a good idea to guard against it, within reason.
There are 'market' reasons against: Every startup requires time to build brand awareness, web presence, connections, etc. Leaving a startup that already has some momentum implies loosing this, and having to start over from scratch.
There are 'reputation' reasons against: This would be especially strong if institutional investors (VCs) are required for the 'cloned' startup -- a call from the old VC team to the new VC team could sink the founders reputation.
There can be / should be legal reasons against: The specifics will vary from country to country, but there are several possible places to attempt to regulate this.
You should go see a lawyer and obtain qualified advice relevant to your legal system and your specific situation. But bottom line, this issue can be addressed with a little bit of forethought.