Display error message when gridview value has duplicate in c#

i have gridview as follows

Selectdata Slipno Slipdate Location Zone Qty

checkbox 123 28.05.2018 1001 Fereke 10
checkbox 123 28.05.2018 1002 Sordi 5
checkbox 100 20.05.2018 1004 Tiepke 12
checkbox 112 22.04.2018 1000 Sordi 11
checkbox 98 20.05.2018 1001 Fereke 15
checkbox 98 20.05.2018 1004 Nord 10

i have one button called submit

suppose when user click slip no 123 using chekcbox in gridview and click submit button i have to show the message to the user "For this slipno one more items is missed to select, please select those items.