While using dedup on a table, splunk will deduplicate values (read cells) of a given field. Your result has a single value/cell (regardless of having multiple values inside it) so there is nothing to dedup.
What you're probably looking for is something like: index=main sourcetype="access_combined_wcookie"| stats values(useragent) as Browsers
index=main sourcetype="access_combined_wcookie"| stats count by useragent
------------ Hope I was able to help you. If so, some karma would be appreciated.