How can i use multiple gridportlets in groupTabPanel

I am facing an issue with using multiple instances of the same gridportlet for multiple tabs in my grouptabpanel example.

I tried creating different instances of gridportlets as well, but that didn't solve the issue

coreWorkFiles.txt

GridPortlet.txt

I also tried creating a simple grid and assigning the grid name to xtype, but that did'nt work as well.

Can you please suggest me any way by which i can have grids in multiple tabs in grouptabpanel. I am using EXTJS 4.1.1.

How can i use multiple gridportlets in groupTabPanel