Following example shows the issue:

Whenever you bind the selectionModel's selectedItem list to another observable list they will eventually contain different/duplicate items. especially when you hit CTL+A and deselect you will notice that the printout sizes don't match.

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package sandbox;

import javafx.application.Application;
import javafx.beans.binding.Bindings;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener;
import javafx.collections.ObservableList;
import javafx.scene.Scene;
import javafx.scene.control.SelectionMode;
import javafx.scene.control.TableColumn;
import javafx.scene.control.TableView;
import javafx.scene.control.cell.PropertyValueFactory;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class TableTest extends Application {
@Override
public void start(Stage primaryStage) {
BorderPane root = new BorderPane();
Scene scene = new Scene(root, 600, 400);

ObservableList<Person> personList = FXCollections.observableArrayList();
Person person1 = new Person("name1", 12);
Person person2 = new Person("name2", 33);
Person person3 = new Person("name3", 44);
Person person4 = new Person("name4", 44);
Person person5 = new Person("name5", 44);
Person person6 = new Person("name6", 44);

personList.setAll(person1, person2, person3, person4, person5, person6);

TableView<Person> personTable = new TableView<>();
personTable.setEditable(true);
personTable.setTableMenuButtonVisible(true);
personTable.setItems(personList);

personTable.getSelectionModel().setSelectionMode(SelectionMode.MULTIPLE);

TableColumn<Person, String> nameColumn = new TableColumn<>("Name");
nameColumn.setCellValueFactory(new PropertyValueFactory<>("name"));

TableColumn<Person, Integer> ageColumn = new TableColumn<>("Age");
ageColumn.setCellValueFactory(new PropertyValueFactory<>("age"));

personTable.getColumns().addAll(nameColumn, ageColumn);

root.setCenter(personTable);

primaryStage.setScene(scene);
primaryStage.show();

ObservableList<Person> _selectedPersons = FXCollections.observableArrayList();

personTable.getSelectionModel().getSelectedItems().addListener((ListChangeListener<Person>) change -> {
System.err.println("selectionModel: " + change.getList().size());
});

_selectedPersons.addListener((ListChangeListener<Person>) change -> {
System.err.println("selectedPersons: " + change.getList().size());
});

Bindings.bindContent(_selectedPersons, personTable.getSelectionModel().getSelectedItems());
}

public static void main(String[] args) {
launch(args);
}

public static class Person {
private StringProperty nameProperty = new SimpleStringProperty();
private IntegerProperty ageProperty = new SimpleIntegerProperty();

public Person(String name, Integer age) {
setName(name);
setAge(age);
}

public StringProperty nameProperty() {
return nameProperty;
}

public void setName(String name) {
nameProperty.set(name);
}

public String getName() {
return nameProperty.get();
}

public IntegerProperty ageProperty() {
return ageProperty;
}

public void setAge(Integer age) {
ageProperty.set(age);
}

public Integer getAge() {
return ageProperty.get();
}

@Override
public String toString() {
return "" + getName() + "/" + getAge();
}
}
}