You may recall the posts on keeping your library clean; here and here. But maintaining a clean, legible, and highly functional BIM file is more than just library management. It is also awareness of the attributes, and keeping those properly organized, sorted, named and vetted.
When creating an attribute it should be named and numbered appropriately for clear function, intent and organization. But attributes can become polluted, just like the libraries. This happens when bringing in downloaded objects which contain their own attributes or new attribute references, or when copying/pasting content from one ARCHICAD file into your working project.
Attributes are almost certainly going to come in in either case. So you can either bring the content in, then fix the libraries and attributes, or you can create a quick sandbox to preview and pre-clean the content.
To do this, simply select file > new, and set the resulting window use the latest project settings and launch in a new instance. This ensures the attributes and settings will match your current project for assessing the damage of copy/paste before traumatizing your project.
Once the content has been pasted or added to the sandbox file, review the embedded library for stray or polluted content, and the attribute manager for content that came in with the content.
In the attribute manager, look for any attributes (typically under lines, fills and surfaces) that have come in with the paste. These will typically show at the bottom of the list when sorting by attribute ID. Look for attributes with big gaps in ID sequence, or attributes underlined or italicized in the case of teamwork projects.
Once these attributes have been removed in the attribute manager, simply search for any content with missing attributes (find & select works wonders for this), and reassign attributes that will copy/paste into your working project file.
This is a little work up front, but will save tons of time on the back side trying to make sense of the project attributes and figuring out which attributes your project needs and which ones came in through sloppy copy/paste practices.