[R-sig-Geo] merging polygons by indices

[majesty]

Dear R-sig-geo subscribers, 

I have the following problem, would be grateful for suggestions. I am doing
a research on UK output areas, My .shp file contains output areas for UK but
there are some duplicates. when I type
which(duplicated(shape$output_area)==TRUE) I get indices of 67 duplicates:

[1] 165713 165723 165724 165786 165788 165814 165815 165819 165820 165821
[11] 165822 165827 165900 165974 165978 165985 165989 166132 166142 166300
 
... and so on

I would like to merge them with the output area polygons of the first
original instance of a given output_area, so that for instance I would merge
16712 with 165713 and 165722 with 165723, etc. 

I know I should use unionSpatialPolygons() function from maptools but how to
do it if I want to merge only particular indices (of which I have a list)?

Any input would be appreciated, 
Thanks, 

Juta

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