fix keep-duplicates help line.
fix keep-duplicates help line.

etataurov/pytest,hpk42/pytest,pytest-dev/pytest,tareqalayan/pytest,davidszotten/pytest,MichaelAquilina/pytest,jaraco/pytest,tgoodlet/pytest,malinoff/pytest,vmalloc/dessert,nicoddemus/pytest,Akasurde/pytest,nicoddemus/pytest,RonnyPfannschmidt/pytest,markshao/pytest,pfctdayelise/pytest,txomon/pytest,ddboline/pytest,eli-b/pytest,tomviner/pytest,hpk42/pytest,The-Compiler/pytest,rmfitzpatrick/pytest,The-Compiler/pytest,flub/pytest,hackebrot/pytest,alfredodeza/pytest,tomviner/pytest,skylarjhdownes/pytest