[R] identify duplicate entries in data frame and calculate mean

[Matthew]

I have a data frame with 10 columns.
In the last column is an alphaneumaric identifier.
For most rows, this alphaneumaric identifier is unique to the file, 
however some of these alphanemeric idenitifiers occur in duplicate, 
triplicate or more. When they do occur more than once they are in 
consecutive rows, so when there is a duplicate or triplicate or 
quadruplicate (let's call them multiplicates), they are in consecutive rows.

In column 7 there is an integer number (may or may not be unique. does 
not matter).

I want to identify each multiple entries (multiplicates) occurring in 
column 10 and then for each multiplicate calculate the mean of the 
integers column 7.

As an example, I will show just two columns:
Length  Identifier
321     A234
350     A234
340     A234
180     B123
198     B225

What I want to do (in the above example) is collapse all the A234's and 
report the mean to get this:
Length  Identifier
337     A234
180     B123
198     B225


Matthew