I wish to export list in sharepoint but the problem is i have 2 columns with the same name in the same library how to choose the second one since the first one is lookup type & it give me an error at the export.
You can always use the column's internal name instead of the column title / display name. The internal name is never duplicate, even when your titles are. Incidentally SharePoint does not let you create duplicate titles either via the web interface, so I assume you got into this with Content Types or Custom List Definitions or some such...
A primer: Columns have two kinds of names, the internal name and the Column title (display name).
The internal name of a Column is set when a it is first created, and it never changes, even though the Column title might be changed repeatedly from SharePoint's Edit Column page later on. The patterns for internal names vary: for some built-in Columns from SharePoint's out-of-the-box List definitions there are internal names such as AssignedTo for the Tasks List's "Assigned To" Column, or _ModerationComments for any List's "Approver Comments" Column. A user-created Column with an initial title of "Test-Link" will get an internal name of Test_x002d_Link.
You'll see the internal Column name in the URL of its Column Settings page, but often it will unfortunately be escaped. Still this may be a way to figure this out. What IS the duplicate title? Maybe we can figure the internal names out for you this way. If you also have FilterZen installed, you can use its column internal name finder tool.