To check for duplicates, you need to check every existing row in the datagridview, from the piece of code, I suspect you check only the current one.
Use the debugger to see exactly what your code is doing.
There is a tool that allow you to see what your code is doing, its name is
debugger. It is also a great learning tool because it show you reality and you can see which expectation match reality.
When you don't understand what your code is doing or why it does what it does, the answer is
debugger.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.
Debugger - Wikipedia, the free encyclopedia[
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Visual Basic / Visual Studio Video Tutorial - Basic Debugging - YouTube[
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Visual Basic .NET programming for Beginners - Breakpoints and Debugging Tools[
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The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.