You can create a browser element the first time you use application browser and then use that element in further use application activities. This would ensure same browser instance is referred everytime.
I am also using library for login, and sending browser element as out argument from library and then using it in rest of workflow.
in use application/browser properties you have a Open property..where you can select if not open or never so that new windows are not opened and it attaches to existing instance with matching name