if worksheet already open then dont open again

I have made two worksheet linked with each other...when i click button "open my sheet" other worksheet automatically open in invisible mode and again if i click button "open my sheet' then a msg appears "you have already open the spec file do u want to reopen again.yes or no"..i want that if my "spec file already open " then dont open again...and the msg does not appear again..

also find the picture of the said msg which i have attached.

please help me out of this problem as soon as possible

thanks in advance..