The 15 character Salesforce ID is NOT case sensitive so when manipulating data in Excel you cannot do VLOOKUP's against a 15 character ID and you cannot use simple commands such as remove duplicates.
Salesforce sees these 2 records as being unique while microsoft programs see them as being the same:
006A000000NoiGl
006A000000Noigl
VLOOKUPs will work, but the data will be COMPLETELY WRONG!
Using something like this tool below you can convert the 15 character ID to the CASE SENSITIVE 18 character ID. Without getting into the logic behind the scenes the above examples convert to the case sensitive IDs below and you CAN do VLOOKUPs and remove duplicates on these.
006A000000NoiGl > 006A000000NoiGlIAJ
006A000000Noigl > 006A000000NoiglIAB
Below is a link to this great tool!
http://www.adminbooster.com/tool/15to18
Alternatively, you can use the CASESAFEID() function in a formula field to convert the ID right in Salesforce.