• Create new empty project
• Add Rectangle to the scene -> It gets id "rectangle1"
• Select "Create component" from navigator view context menu for rectangle1
• Suggested name is "MyRectangle1"
• Click OK

--> rectangle1 is converted into a MyRectangle1 component

• Delete rectangle1
• Add another Rectangle from components view -> It gets id "rectangle1"
• Select "Create component" from navigator view context menu for rectangle1
• Suggested name is "MyRectangle1" (I.e. same as already existing component)
• Click OK

--> MyRectangle1 component already exists, so nothing happens. There probably should be a warning or something, or maybe OK should be disabled so you can't even try it?

You can create duplicate MyRectangle1 if you uncheck the "ui.qml file" checkbox on the second creation; then you end up with MyRectangle1.qml and MyRectangle1.ui.qml, which probably will cause some issues.