Remove the duplicated history items from `peco` selection.

```
diff --git a/.zshrc b/.zshrc
index 22d417a..74c11f6 100644
--- a/.zshrc
+++ b/.zshrc
@@ -79,6 +79,7 @@ if which peco > /dev/null; then

     BUFFER=$(history -n 1 | \
       eval $tac | \
+      awk '!a[$0]++' | \
       peco --query "$LBUFFER")

     CURSOR=$#BUFFER
```