You might reconsider the LEFT function (for a text string located in A1, change as needed):
=IFERROR(LEFT(SUBSTITUTE(A1;LEFT(A1;FIND("/";A1));"");FIND("/";SUBSTITUTE(A1;LEFT(A1;FIND("/";A1));""))-1);"")
It will return nothing if there is no "/" in the text string, or if there is only one "/". Else, it will return the text between the first two slashes...
To eliminate duplicates, just copy/paste as values the results of the formulas, and use Data-remove duplicates.