How to UnDefine a variable in SqlPlus?

Hi all,
 I executed a script from sqlplus which included a && prameter.I executed completely.Now I want to execute it for second time but it does not free that && variable and do not ask for it from me.  I cleared buffer but no chance.What should I do to undefine this in the same session?

 Note: I NEEDed that to be && in original scripts because of many times of using it :)

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