title: error in plm regression
body: <p>colleagues! I have panel data:</p>
  
  <pre><code>    Company year       Beta     NI   Sales  Export Hedge      FL     QR     AT Foreign
  1       1 2010 -2.2052800 293000 1881000 78.6816     0 23.5158  1.289 0.6554    3000
  2       1 2011 -2.2536069 316000 2647000 81.4885     0 21.7945 1.1787 0.8282   22000
  3       1 2012  0.3258693 363000 2987000 82.4908     0 24.5782 1.2428  0.813  -11000
  4       1 2013  0.4006030 549000 4546000 79.4325     0 31.4168 0.6038 0.7905   71000
  5       1 2014 -0.4508811 348000 5376000 79.2411     0 37.1451 0.6563  0.661  -64000
  6       1 2015  0.1494696 355000 5038000 77.1735     0 33.3852 0.9798 0.5483   37000
  </code></pre>
  
  <p>But R shows the mistake when I try to use plm package for the regression:</p>
  
  <pre><code>panel &lt;- read.csv("Panel.csv",  header=T, sep=";")
  p=plm(data=panel,Beta~NI, model="within",index=c("id","year"))
  
  
  Error in pdim.default(index[[1]], index[[2]]) : 
    duplicate couples (id-time)
  In addition: Warning messages:
  1: In pdata.frame(data, index) :
    duplicate couples (id-time) in resulting pdata.frame
   to find out which, use e.g. table(index(your_pdataframe), useNA = "ifany")
  2: In is.pbalanced.default(index[[1]], index[[2]]) :
    duplicate couples (id-time)
  
  3: In is.pbalanced.default(index[[1]], index[[2]]) :
    duplicate couples (id-time)
  </code></pre>
  
  <p>I searched this error in the Internet and read that it's connected with the id of company and year. But I did not find the way how to avoid this problem. Also, when I do na.omit(panel), R does not show the error, but it's significant to stay NA data and companies in the data. Please, tell me to do with this problem. Thank you.</p>