diff --git a/views/application.coffee b/views/application.coffee
index 7374176..c800384 100644
--- a/views/application.coffee
+++ b/views/application.coffee
@@ -87,6 +87,18 @@ class Nikoli.Cell
     0 <= @x < @game.length && 0 <= @y < @game[@x].length &&
       (!value? || value < 0 && @game[@x][@y] < 0 || value >= 0 && @game[@x][@y] >= 0)
 
+  duplicatesIn: (array) ->
+    array.filter((cell) => cell == @value).length > 1
+
+  columnDuplicates: ->
+    column = []
+    column.push @game[i][@y] for i in [0...@game.length]
+
+    @duplicatesIn column
+
+  rowDuplicates: ->
+    @duplicatesIn @game[@x]
+
 class Nikoli.Stream
   constructor: (@game) ->
     @cells = []
diff --git a/views/hitori.coffee b/views/hitori.coffee
index 82f558c..08aca82 100644
--- a/views/hitori.coffee
+++ b/views/hitori.coffee
@@ -18,7 +18,9 @@ class Nikoli.Hitori extends Nikoli.Game
 
           if !white_stream.include(cell)
             errors.push {row: i, column: j, message: 'The stream must be continuous'}
-          # TODO check for duplicates in rows and columns
+
+          if cell.rowDuplicates() || cell.columnDuplicates()
+            errors.push {row: i, column: j, message: 'The number appears more than once in the row or column.'}
         else
           if cell.adjacentCells().some((adj_cell) -> adj_cell.valid(-1))
             errors.push {row: i, column: j, message: 'Adjacent filled-in cells'}