[R] collapse rows in a matrix

[yiping.fan@syngenta.com]

Hi, all,
  I have a little problem to solve. I'd like to collapse rows which are next
to each other but have
 same value to one row. The following is an example.

say x is a data frame like:
   X1 X2 X3 X4 X5
a  1  0  0  0  1
b  1  0  1  0  1
c  1  0  0  0  1
d  1  0  0  0  1
e  1  0  0  0  1
f  1  1  0  0  1
g  1  1  0  0  1

notice that  a, c,d,e are the same. since c,d,e are next to each other, I
will only use the middle
one,i.e d.   I will also  keep a although it is the same as d.

f,g , I will keep f or g.

so the ideal output is: 
a  1  0  0  0  1
b  1  0  1  0  1
d  1  0  0  0  1
f  1  1  0  0  1

Any idea how to do it?  Thanks!!

Y. Fan
by the way,
unique(x) does not give me the desired output:  it remove c,d,e
  X1 X2 X3 X4 X5
a  1  0  0  0  1
b  1  0  1  0  1
f  1  1  0  0  1



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