concat produces incorrect output
Under certain circumstances, concat seems to produce erroneous results. I haven't worked out what causes the problems to arise, but here's an example:

df1 = DataFrame({'firmNo' : [0,0,0,0], 'stringvar' : ['rrr', 'rrr', 'rrr', 'rrr'], 'prc' : [6,6,6,6] })
df2 = DataFrame({'misc' : [1,2,3,4], 'prc' : [6,6,6,6], 'C' : [9,10,11,12]})
concat([df1,df2],axis=1)

produces as output:

  firmNo  prc  stringvar   C  misc  prc
0    rrr    0          6   9     1    6
1    rrr    0          6  10     2    6
2    rrr    0          6  11     3    6
3    rrr    0          6  12     4    6
