[BioC] Affy genechips: probes many times

[Saroj Mohapatra]

Hi friends:

I find that there are multiple instances of probes (i.e., the same 
25-mer sequence) on the same array.

For example:
--------------

 > library("hgu133atagprobe")
 > seqnc <- hgu133atagprobe$sequence

-------------
There are 248152 probes, out of which 6067 are duplicated (the number 
varies between 2 and 20).
-------------

 > length(seqnc)
[1] 248152
 > sum(duplicated(seqnc))
[1] 6067
 > numTimes = sapply(unique(seqnc[duplicated(seqnc)]), function(x) 
sum(seqnc==x))
 > range(numTimes)
[1]  2 20

-------------

I have the question in mind:  what is the logic behind multiple 
instances of the same probes (e.g., quality control?)

I thank in advance for any feedback or pointer to literature.

Saroj