After further analyzation of this issue I came to following insights:

When the error of duplicate users occurs, the participants that the app tried to write to mongo seems to be fully correct with no duplicate users.
However it appears there always happen 2 nearly synchronous updates of the participants list, each trying to add the new participants. It seems this leads to a chance of Mongo messing up the minutes' document and add users twice.

This synchronous calls seem to occur due to Meteor's optimistic-UI. Looking at the function call order this gets clear:

Code starts within the MeetingSeries-Editor-Template
Within the MeetingSeries class the function setVisibility is called
Within the Minute class the function SyncVisibility is called
Now the Meteor Method syncVisibility is called.
Due to the optimistic UI all of the following code is executed both on server and client.
The Minute class's function refreshParticipants is called
The Minute class's function update is called
Finally the Meteor Method minute.update is called, in which the MongoCollection is updated. Hence the OptimisticUI this Meteor-Method-Call is done both by server and by client. This leads to the server trying to execute this method 2 times parallel.

This last duplicate call seems to trigger the error.
By surpressing the redundant call from the server the error seems to disappear:

if (Meteor.isClient) { await Meteor.callPromise ('minutes.update', docPart, callback); }

In order to reproduce the error you can try to run the following e2e-Test

`import { E2EGlobal } from './helpers/E2EGlobal';
import { E2EApp } from './helpers/E2EApp';
import { E2EMeetingSeries } from './helpers/E2EMeetingSeries';
import { E2EMinutes } from './helpers/E2EMinutes';
import { E2EMeetingSeriesEditor } from './helpers/E2EMeetingSeriesEditor';
import { E2EMinutesParticipants } from './helpers/E2EMinutesParticipants';

describe('MeetingSeries', function () {
beforeEach("goto start page and make sure test user is logged in", function () {
E2EApp.gotoStartPage();
expect (E2EApp.isLoggedIn()).to.be.true;
});

before("reload page and reset app", function () {
    E2EApp.resetMyApp(true);
    E2EApp.launchApp();
});

it('DuplicateUsers-TEST-SERIES @watch', function () {
    //This test will create a MS and a Minute as User1
    //Then it will iterate the following steps: Add User2 as informed, promote User2 to invited, remove User2
    this.timeout(6000000);
    let aProjectName = 'DEKA';
    let aMeetingName = 'DEKA';
    E2EMeetingSeries.createMeetingSeries(aProjectName, aMeetingName);
    let msid = E2EMinutes.addMinutesToMeetingSeries(aProjectName, aMeetingName);

    let counter = 0;
    let user2 = E2EGlobal.SETTINGS.e2eTestUsers[1];
    while(user2) {
        //User2: Informed
        E2EMeetingSeriesEditor.openMeetingSeriesEditor(aProjectName, aMeetingName, "invited");
        E2EMeetingSeriesEditor.addUserToMeetingSeries(user2, E2EGlobal.USERROLES.Informed);
        E2EMeetingSeriesEditor.closeMeetingSeriesEditor();  // close with save
        //User2: Invited
        E2EMeetingSeriesEditor.openMeetingSeriesEditor(aProjectName, aMeetingName, "invited");
        let roleSelector = "select.user-role-select";
        browser.selectByValue(roleSelector, E2EGlobal.USERROLES.Invited);
        E2EMeetingSeriesEditor.closeMeetingSeriesEditor();  // close with save
        //Check Error Occured
        let min = server.call('e2e.findMin',msid);
        let partcount = min.participants.length;
        if (partcount !== 2) { //Error happened
            counter += 1;
            console.log('____________________________________________' + counter);
            console.log(min.participants);
            console.log('____________________________________________');
        }
        console.log('BISHER #FEHLER:' +  counter);

        //User2: Remove
        E2EMeetingSeriesEditor.openMeetingSeriesEditor(aProjectName, aMeetingName, "invited");
        let usersAndRoles = E2EMeetingSeriesEditor.getUsersAndRoles(0,1,2);
        browser.elementIdClick(usersAndRoles[user2].deleteElemId);
        E2EMeetingSeriesEditor.closeMeetingSeriesEditor();  // close with save
    }
});

});
`

I let this test run for arround 2 hours, the error occured 14 times. With the surpression of the redundant call it did not appear in 1.5 hours once.
(This test can be run both in GUI and in headless mode. Errors occur on both ways)

Although the above shown surpression of the redundant call would fix the issue, this is of course more of a quick&dirty hotfix.
Furthermore until now I haven't analyzed from which locations the minute class's update function is currently called. If there would be for example a code piece only running on the server and using the update-function it would cause errors to allow the meteor method call only on client.
Therefore this issue needs further inspection to find the best way to deal with it.

Meeting Series "Invited Editor" => sometimes adds user more than once to active minutes #293

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