I have some custom buttons that I want to keep around for SF Classic but in Lightning I want them to show up as Lightning Experience Actions. I created separate Actions for them but now there are duplicates. I want to hide the old custom buttons in Lighting so users don't see both. Is there a way to hide these buttons in Lightning but keep them in Classic view?
Yes, Whenever you create a custom button on an object, it does not come on the layout itself. You need to add in the detail section on the layout in the custom button area. When you do that, it applies for classic only.
In order to show a custom button in Lightning, You need to add the custom button in Salesforce Mobile and Lightning Experience Actions section. If you don't do that, the custom button will not appear in Lightning.
As per your need, You need to add the custom button in detail section in custom button area and add the action which you have created for lightning in Salesforce Mobile and Lightning Experience Actions section.
Refer to the image below :-