Removed duplicates keywords

Can be found with the following python snippet

import re
plumedDictionary={}
with open("/opt/local/lib/plumed/vim/syntax/plumed.vim") as fin:
    for line in fin:
        if re.match("^let b:plumedDictionary\[.*$",line):
            line=re.sub("^let b:","",line)
            exec(line,{'__builtins__': None},{'plumedDictionary':plumedDictionary})
for key in plumedDictionary.keys():
    diction={}
    for opt in plumedDictionary[key]:
        if(opt['word'] in diction):
            print(key,opt['word'])
        diction[opt['word']]=1