html5 help: sts exports duplicates (errors) in css

Hi there,
when I'm publishing (FrameMaker 2019, 15.0.5.838) a html5 help from an fm-book, the produced css-file always lists both the "old fm-formats" (which I changed/allocated in the sts-file) and the new formats for web publishing.
So that there are always many duplicates (errors in the css), see example.
If I clean up the tags and re-import the (cleaned) css file into the sts, save the sts --> the next publishing export will again have the duplicates!
Where/how can I force the sts-file to only use my cleaned, imported css and not to publish the old (imported fm-)styles together with the new styles (allocated in the sts)?

Example with duplicate styles:
p.FM_Text {

margin-left:14.173pt;

margin-right:0.000pt;

text-align:left;

text-indent:0.000pt;

margin-top:0.000pt;

margin-bottom:4.000pt;

word-spacing:0.250em;

font-family:"Rotis Sans Serif W1G";

font-style:normal;

font-weight:normal;

font-size:11.0pt;

color:black;

background-color:none;

text-decoration:none;

letter-spacing:0.00em;

margin:0pt 0pt 10pt 90pt;

font:300 normal 10pt Open Sans Light, Calibri, Arial, sans-serif;

}

Thanks,
Eva