uniq: add page

pages/common/uniq.md

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+# uniq
+
+> Report or omit repeated lines.
+
+- Display each line once:
+
+`uniq {{file}}`
+
+- Display only unique lines:
+
+`uniq -u {{file}}`
+
+- Display only duplicate lines:
+
+`uniq -d {{file}}`
+
+- Display number of occurences of each line along with that line:
+
+`uniq -c {{file}}`