I am in the process of trying to optimize an external HD and it tells me that I have LR5 preview lrdata that have like 20 duplicates. Is there a way that I can determine if in fact 2 of the files ( or more) are in fact duplicates? Can I import these files into LR CC? How do I know which files are safe to delete and if I do, will that effect the ability for LR to read the data of those that remain?
I am in the process of trying to optimize an external HD and it tells me that I have LR5 preview lrdata that have like 20 duplicates.
Please be more specific. Exactly what are you doing? What do you see? What is the exact message?
What exactly does "optimize an external HD" mean, and why do you need to do this?
If you are concerned that your previews are taking too much space, you can in Lightroom, delete the 1:1 previews, however this is a tradeoff as then Lightroom will work slower as then Lightroom has to create previews on the fly.
Whatever you choose to do with regards to the previews, do it in Lightroom and not via any other method.
You must consider the Previews.LRDATA folder to be a software "cache".
Basically it does not contain photo files that you can copy or delete. The previews are fully under the control of Lightroom.
By 'optimize' you probably mean 'Defrag'. Ignore anything about duplicates in the Previews folder.
The Previews.lrdata folder can be totally deleted, but see DJ's comment "this is a tradeoff....."