duplicate import paths

[SingingBush]

I have a dcd.conf with /usr/include/dmd/druntime/import and /usr/include/dmd/phobos so starting dcd-server with simply dcd-server is enough to get going. When it comes to integrating DCD into an IDE however it's easier to start the server with dcd-server -I/usr/include/dmd/druntime/import -I/usr/include/dmd/phobos so we don't need to worry too much about dcd.conf but I've noticed in the servers log output that it doesn't filter the duplicated paths:

2019-01-06T10:20:02.974:main.d:main:117 Starting up...
2019-01-06T10:20:02.974:server.d:loadConfiguredImportDirs:94 Loading configuration from /home/samael/.config/dcd/dcd.conf
2019-01-06T10:20:02.975:main.d:main:144 Cleaning up old socket file at /run/user/1000/dcd.socket
2019-01-06T10:20:02.975:main.d:main:151 Listening at /run/user/1000/dcd.socket
2019-01-06T10:20:03.430:first.d:visit:428 Could not resolve location of module 'gcc/builtins'
2019-01-06T10:20:04.301:first.d:visit:428 Could not resolve location of module 'ldc/intrinsics'
2019-01-06T10:20:04.916:main.d:main:168 Import directories:
    /usr/include/dmd/druntime/import
    /usr/include/dmd/phobos
    /usr/include/dmd/druntime/import
    /usr/include/dmd/phobos
2019-01-06T10:20:04.916:main.d:main:174 114115 symbols cached.

At this stage it's not too much of a problem as it's just phobos and druntime but there's a potential for this to get out of hand as we start adding import paths for the various dub dependencies that a project may use. It should be simple enough for DCD not to add a path if it's already tracked so that users don't have to worry about keeping track of which paths have already been added to dcd.