Excellent question, there are many ways to duplicate items in SoftPlan. In your scenario where you are designing a multi-unit complex, Softplan has the ability to create Nested Regions to where you can duplicate a single drawing and have any changes affect any of the copied instances (useful for duplexes, apartment complexes, town homes, etc. all with the same floor plan design). I would recommend watching this official SoftPlan video that is about 5 minutes long and explains a useful tool in your scenario: (it is worth the 5 minutes )
(184) SoftPlan Nested Regions - YouTube
However, there is a few other manual tools to copy objects within SoftPlan. Under 'Tools' called 'Group Box' is used to group items all together and allowing them to duplicate as a group. Select the tool 'Group Box' and place a selection around your desired items. It will look like it did nothing, but it is essential in the next steps. Next, you can use the tool under 'Move' called 'Copy'. Select the 'Copy' Tool (c for shortcut) and make sure before you click on one of the grouped items to copy, enable Group in the top ribbon (near the 'Attach' toggle). Right click an object in your initial 'Group' selection and when moving the selection to your desired copy location, you will see the objects selected in the group (ensuring you have 'Group' enabled in the top ribbon) will be copied along the selection and placed when you right click again to finalize. This grouping property also works with 'Move' and 'Align'. Before using any of these three tools, make sure you want either 'Attach' and especially 'Grouped' enabled since doing any of these tasks will affect any objects in a group if 'Group' is enabled in the ribbon.
Another tool is 'Copy Block' under 'Move' and the tool named 'Copy Block'. This works similar to grouping items in the initial method, but it copies under a box selection to the desired location.
Your scenario more than likely would be useful to explore Nested Regions as explained in the video since SoftPlan released this feature for your specific purpose. All of the other tools explained are manual methods and can still be useful for copying grouped items, but exploring Nested Regions is a great way to be efficient with your project. Hopefully that helps and gets you started a little bit.