need algorith to combine sets

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Hi I have to lists, using C# and the look like
List <stringreportna me = new List<string>();
List <Int32 reportnumber = new List <Int32>();
They get filled with corresponding data, both lists are the same size. I
need to create a third string lists and fourth integer list that combines the
report names when the report numbers are the same. For example the data
might look like
reportname report number
vehicles 1
bikes 1
cities 2

so I would like to create a third and fourth list that would have
report name report number
vehicles, bikes 1
cities 2
thanks.

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Paul G
Software engineer.