title: Duplicated rows when I merge 2 data frame
body: <p>This is Fips data set</p>
  
  <pre><code>   State Fips State.Abbreviation ANSI.Code        GU.Name
  1      1   67                 AL   2403054      Abbeville
  2      1   73                 AL   2403063     Adamsville
  3      1  117                 AL   2403069      Alabaster
  4      1   95                 AL   2403074    Albertville
  5      1  123                 AL   2403077 Alexander City
  6      1  107                 AL   2403080     Aliceville
  7      1   39                 AL   2403097      Andalusia
  8      1   15                 AL   2403101       Anniston
  :
  :
  :
  41774    51  720                 VA   1498434           Norton
  41775    51  730                 VA   1498435       Petersburg
  41776    51  735                 VA   1498436         Poquoson
  41777    51  740                 VA   1498556       Portsmouth
  41778    51  750                 VA   1498438          Radford
  41779    51  760                 VA   1789073         Richmond
  41780    51  770                 VA   1498439          Roanoke
  41781    51  775                 VA   1789074            Salem
  41782    51  790                 VA   1789075         Staunton
  41783    51  800                 VA   1498560          Suffolk
  41784    51  810                 VA   1498559   Virginia Beach
  41785    51  820                 VA   1498443       Waynesboro
  41786    51  830                 VA   1789076     Williamsburg
  41787    51  840                 VA   1789077       Winchester
  
  dim(fips)
  [1] 2937    5
  </code></pre>
  
  <p>This is data head cancer</p>
  
  <pre><code>   PUBCSNUM  REG MAR_STAT RACE1V NHIADE SEX FIPS Fips State State.Abbreviation
  1  93261752 1544        2     15      0   1    3    3    34                 NY
  2  93264865 1544        2      1      0   1   15   15    34                 NY
  3  93268186 1544        2      1      0   1    5    5    34                 NY
  4  93272027 1544        2      1      0   2   17   17    34                 NY
  5  93274555 1544        1      1      0   1   13   13    34                 NY
  6  93275343 1544        5      1      0   2   25   25    34                 NY
  7  93279759 1544        5      1      0   2    9    9    34                 NY
  8  93280754 1544        2      1      0   2   35   35    34                 NY
  9  93281166 1544        2      1      0   2   31   31    34                 NY
  10 93282602 1544        5      1      0   1   33   33    34                 NY
  11 93287646 1544        1      1      0   1   11   11    34                 NY
  12 93288255 1544        4      1      4   1   39   39    34                 NY
  13 93290660 1544        9      1      0   2   25   25    34                 NY
  14 93291461 1544        1      1      6   1   39   39    34                 NY
  15 93291778 1544        2      1      0   1    3    3    34                 NY
  
  dim(headcancer)
  [1] 75313    10
  </code></pre>
  
  <p>when I merged together I expect to get the same row with head.cancer 75313 rows, but I got 951423 rows.</p>
  
  <p>Here is my code and output</p>
  
  <pre><code>n = merge(head.cancer,fips, by=c('State','Fips','State.Abbreviation'), all.x= TRUE)
  
     State Fips State.Abbreviation PUBCSNUM  REG MAR_STAT RACE1V NHIADE SEX FIPS ANSI.Code            GU.Name
  1      6    5                 CA 70128269 1541        4      1      0   2    5   2409693        Amador City
  2      6    5                 CA 70128269 1541        4      1      0   2    5   2411446           Plymouth
  3      6    5                 CA 70128269 1541        4      1      0   2    5    226085            Jackson
  4      6    5                 CA 70128269 1541        4      1      0   2    5   1675841             Amador
  5      6    5                 CA 70128269 1541        4      1      0   2    5   2418631 Ione Band of Miwok
  6      6    5                 CA 70128269 1541        4      1      0   2    5   2412019       Sutter Creek
  7      6    5                 CA 70128269 1541        4      1      0   2    5   2410110               Ione
  8      6    5                 CA 70128269 1541        4      1      0   2    5   2410128            Jackson
  9      6    5                 CA 67476209 1541        2      1      1   2    5   2409693        Amador City
  10     6    5                 CA 67476209 1541        2      1      1   2    5   2411446           Plymouth
  11     6    5                 CA 67476209 1541        2      1      1   2    5    226085            Jackson
  12     6    5                 CA 67476209 1541        2      1      1   2    5   1675841             Amador
  13     6    5                 CA 67476209 1541        2      1      1   2    5   2418631 Ione Band of Miwok
  14     6    5                 CA 67476209 1541        2      1      1   2    5   2412019       Sutter Creek
  15     6    5                 CA 67476209 1541        2      1      1   2    5   2410110               Ione
  16     6    5                 CA 67476209 1541        2      1      1   2    5   2410128            Jackson
  17     6    5                 CA 56544761 1541        4      1      0   2    5   2409693        Amador City
  18     6    5                 CA 56544761 1541        4      1      0   2    5   2411446           Plymouth
  19     6    5                 CA 56544761 1541        4      1      0   2    5    226085            Jackson
  20     6    5                 CA 56544761 1541        4      1      0   2    5   1675841             Amador
  
  dim(n)
  [1] 951423     12
  </code></pre>
  
  <p>The first row to 8th  "PUBCSNUM "duplicate 8 times, "PUBCSNUM" is ID, so it's unique, "ANSI.Code" is supposed only 1 value, now they are so many value.I don't know why it's duplicate like that</p>
  
  <p>Please help me, I stuck for couples hours but I couldn't figure out. Thanks</p>