If you want to delete duplicate lines throughout the whole file, just do the following:

:g/any part of the string/d

Example:

In command mode

:g/I want to delete/d

If you just want a list of the string(s) initially:

:g/I want to delete/p

If you want to delete duplicate lines throughout the whole file, just do the following:

:g/any part of the string/d

Example:

In command mode

:g/I want to delete/d

If you just want a list of the string(s) initially:

:g/I want to delete/p