We often have this problem too, but it is easy to fix. Remember many VFP modules have 2 files? For example, form have 2 files (.SCX, .SCT). Undes VSS they have 3 files. When one of these files checked in and another is not checked in - you will have this error, and no way to check/uncheck module in VFP.
To fix it, open Visual Source Safe native application and uncheck/check needed files manually.

>I am currently trying to incorporate a large project into source safe and am having real problems. Firstly I added a project to source safe everything seemed fine, then i tried to give another developer access to the source throught the 'join source control project' option under the files menu. Seemed fine until it started to copy to the local drive where an error occured 'SCC API ALREADY CHECKED OUT OCCURED' and further explained that maybe the multiple checked out option was not enabled, unfortunatlly it was checked in source safe (version 6.0 of VFP and 6 of VSS.
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>Another quiestion, is it possible to merge branches with fox files in particular forms and classes. waht I mean by this if i have 2 versions of my source, 1 current release and another the version before and i find out that a feature that worker previously does not work now i want to go back to the last release sources find the differences and merge the last into the current release without overidding and to flow down to any branched projects, without over riding the changes i might have made with current development.
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>I have tried to nut this one out but have come up short on searching for help.
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>TIA