"No QSOs were processed because some QSOs were duplicates or invalid"
In the context of submitting QSOs to LoTW, the term duplicates means QSOs already submitted to LoTW. If you manually populate the QSL Queue, or if you manually select a set of QSOs in the Log Page Display and then direct DXKeeper to submit these to LoTW, you may include QSOs that have already been submitted to LoTW, and have not changed (in a way relevant to LoTW) since they were submitted. Submitting QSOs to LoTW from an application other than DXKeeper can result in DXKeeper attempting to submit QSOs to LoTW that it doesn't know have already been submitted.
The presence of such duplicate QSOs in a batch of QSOs submitted to LoTW will caused all QSOs to be rejected. Why? Because before duplicate checking was implemented, users of less sophisticated logging applications were submitting every QSO in their log each time they had a few new QSOs to submit; the load placed on the LoTW Server by these unnecessary submissions was significant.
To avoid submitting already-submitted QSOs to LoTW, use DXKeeper's automation, which populates the QSL Queue will QSOs not yet submitted to LoTW.
If after manually populating the QSL Queue or manually selecting a set of QSOs in the Log Page Display, your attempt to submit them to LoTW results in DXKeeper displaying the "No QSOs were processed because some QSOs were duplicates or invalid" message, and if the total number of QSOs involved is small - less than 100 - then you override the duplicate detection by checking the Permit uploading of QSOs already uploaded to LoTW box on the QSL Configuration window's LoTW tab. Then repeat your attempt to submit QSOs to LoTW.
If there are more than 100 duplicate QSOs, then update your logged QSOs to reflect their acceptance by LoTW.
On the Main window's Log QSOs" tab, click the Filter panel's X button so that all QSOs will be present in the Log Page Display
On the Main window's QSL tab,
if the QSL Queue is populated, either complete the QSLing operation already in progress, or click the Clear button to clear the QSL Queue
set the QSL Via panel to LotW
while depressing the CTRL key, click the Sync LoTW QSOs button
Step 3 will direct LotW to report all QSOs already submitted to and accepted by LoTW; DXKeeper will set each such QSO's QSL Sent item to Y to reflect acceptance. Depending upon how many QSOs you've submitted to LoTW, this operation can take a long time, but when it's complete, your logged QSOa will accurately reflect their status; DXKeeper's Add Requested and Add Needed functions will not submit QSOs that have already been submitted.