如何在R中给出单词列表中找到唯一单词

I have a list of words

我有一个单词列表

a = c("when","to","use","each","effect","","recognizing","each","effect","?",":")
a
[1] "when"        "to"          "use"         "each"       
[5] "effect"      ""            "recognizing" "each"       
[9] "effect"      "?"           ":" 
a = c("when