Fixes #323.

src/nimble.nim

@@ -455,10 +455,10 @@ proc installFromDir(dir: string, requestedVer: VersionRange, options: Options,
     # Set file permissions to +x for all binaries built,
     # and symlink them on *nix OS' to $nimbleDir/bin/
     for bin in pkgInfo.bin:
-      if not existsFile(pkgDestDir / bin):
+      if existsFile(pkgDestDir / bin):
         display("Warning:", ("Binary '$1' was already installed from source" &
                             " directory. Will be overwritten.") % bin, Warning,
-                HighPriority)
+                MediumPriority)
 
       # Copy the binary file.
       filesInstalled.incl copyFileD(pkgInfo.getOutputDir(bin),