xorg-libXdmcp: /opt/local/lib/libXdmcp.6.dylib already exists and does not belong to a registered port

[trash@…]

$ sudo port install gnuplot
--->  Computing dependencies for gnuplot
--->  Dependencies to be installed: gd2 xpm xorg-libXext xorg-libX11 xorg-libXdmcp xorg-libxcb python27 db46 db_select libedit openssl python_select sqlite3 xorg-libpthread-stubs xorg-xcb-proto libxml2 xorg-xextproto xorg-libXt xorg-libsm xorg-libice libcerf lua readline pango Xft2 xrender xorg-renderproto cairo glib2 libffi perl5 perl5.16 gdbm libpixman xorg-xcb-util gobject-introspection libtool harfbuzz graphite2 pkgconfig wxWidgets-3.0 wxWidgets_select
--->  Activating xorg-libXdmcp @1.1.1_0
Error: org.macports.activate for port xorg-libXdmcp returned: Image error: /opt/local/lib/libXdmcp.6.dylib already exists and does not belong to a registered port.  Unable to activate port xorg-libXdmcp. Use 'port -f activate xorg-libXdmcp' to force the activation.
Error: Failed to install xorg-libXdmcp
Please see the log file for port xorg-libXdmcp for details:
    /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_x11_xorg-libXdmcp/xorg-libXdmcp/main.log
Error: The following dependencies were not installed: gd2 xpm xorg-libXext xorg-libX11 xorg-libXdmcp xorg-libxcb python27 db46 db_select libedit openssl python_select sqlite3 xorg-libpthread-stubs xorg-xcb-proto libxml2 xorg-xextproto xorg-libXt xorg-libsm xorg-libice libcerf lua readline pango Xft2 xrender xorg-renderproto cairo glib2 libffi perl5 perl5.16 gdbm libpixman xorg-xcb-util gobject-introspection libtool harfbuzz graphite2 pkgconfig wxWidgets-3.0 wxWidgets_select
To report a bug, follow the instructions in the guide:
    http://guide.macports.org/#project.tickets
Error: Processing of port gnuplot failed

Already tried:

$ sudo port clean --all all
$ sudo port -f uninstall inactive

[ryandesign (Ryan Carsten Schmidt)]

But why does /opt/local/lib/libXdmcp.6.dylib already exist but not belong to a registered port? How did that file get there, if not via MacPorts?

You can force the activation of xorg-libXdmcp, but if you can't explain why one of its files was already on your disk, you may have other similar problems in the future with other ports, and should probably ​uninstall MacPorts (and completely remove /opt/local) to ensure all rogue files are purged; then you can reinstall MacPorts and the ports you want and shouldn't run into these problems again.

[trash@…]

After complete uninstall and reinstall everything works fine. The first time the installation of gnuplot was started I seem to remember that there was a problem with Xcode. So the fragments in opt/local/ might be from that attempt. Thank you very much for your help!