I'm having trouble with duplicating vector layers in QGIS 3.4. Each time I try and duplicate a layer, the layers do not act independently of one another and if I make changes to one layer, the changes also apply to the copy/original layer. How do you duplicate layers so they act independently? Do you instead need to select features and create a new shapefile layer? This seems odd as there are instructions in QGIS 3.4 for duplicating layers which act independently! I must be doing something wrong...
QGIS' Duplicate layer feature will create another instance of the same base data that acts independently in terms of styling! Changes to the base data (the shapefile) will be present in all instances of that data.
If you want to duplicate the base data, either save as file, or duplicate the file in your OS and load separately.
Alternatively, you can work with the DB Manager / Virtual Layers to create in-memory layer from your data that act as (temporary) independent data sources.