Flex Library Project - Compiler setting "-target-player=10" giving error


I have a Flex Library Project, which uses Vector class (instead of Array). So I am wanted to use minimum Flash Player as 10. But when I set the same in “Additional Compiler Arguments” as “-target-player=10” I get the following compile time error:

[COLOR=Red]“configuration variable ‘target-player’ must only be set once”[/COLOR]

Can anyone tell me how to avoid that?

Here is the steps I am following

Goto Project -> Rt. Click -> Properties -> Flex Library Compiler
There under “Additional Compiler Arguments” set the param as “-target-player=10” (without quotes).