Copy and Paste has a lot of different contexts so it isn't just simply a matter of copy and paste, but rather what you are wanting to copy and paste. You can copy and paste entire drawings (cells) or you can copy and paste component parts of complete drawing (drawing objects) or you can copy and paste selective key framed parameters. And in addition to the various contexts for copy and paste there is also duplicating for cells which isn't the same as a copy and paste.

So how do we sort all this out? First we need to be clear that a cell is a complete drawing. An element is a stack of drawings (one or more cells). Each of those cells can be assigned to one or more frames for that element in a time line sequence. The assignment of a cell to a frame is called an exposure. A cell is not a frame, a frame is not a cell. There is only one of each frame that exists in a time sequence, but there can be many instances of the same cell exposed in that same time sequence. So each frame is numbered to identify its time based location in the sequence. And each cell is uniquely named so that it can be identified each time it is exposed (assigned to a specific frame). When you copy a cell and paste that copy to a new frame for the same element then you have just actually assigned the same identical cell to an additional frame. It isn't a separate copy it is the same exact cell, it just has been assigned an additional exposure. In the exposure sheet adjacent frames that have the same cell assigned to them are indicated by a line through the frame much like you made a "ditto" notation on a piece of paper to show that the word on the line above is repeated on the next line and so on.

If you want to have a unique instance of a cell you don't copy and paste the cell you duplicate the cell.

The easiest way to duplicate a cell is as follows.
(1) select the cell you want to duplicate in the exposure sheet or in the timeline.
(2) right click to open the context menu.
(3) select the "add exposure" command (keyboard short cut R)
(4) move to the frame where the added exposure was created and select the new exposure of your original cell.
(5) right click to open the context menu.
(6) select the "duplicate drawing" command.
You now have an exact uniquely named duplicate of your original cell.

A different type of duplication:
If you select a cell in element "A" and you drag it to a frame position in a new element "B" on your exposure sheet (or timeline) the cell will be renamed to fit the naming order for element "B" and it will no longer be assigned to the frame it occupied in element "A", but the originally named version of the cell still exists in element "A", it just no longer has a current frame assignment. So it appears to disappear. You see a cell is a drawing and it exists in the element where it was created even if it is no longer assigned a time sequence frame position for that element.

If you select a frame for element "A" and you go to the "cells" panel and type in the number for the "missing" cell, it will be there and will be reassigned to that frame in addition to being a unique cell in element "B".

Because the same cell is part of two separate elements it has two unique names and therefore each instance of that cell is unique and separate from the instance in the other element. Effectively you duplicated the cell in a different element by dragging the cell from one element to another element.

What about other form of copy and paste? Let's copy a drawing object. First we need to be sure we are clear on what is a drawing object. A cell is a drawing and a drawing is composed of one or more drawing objects. So if you have a cell displayed in drawing view you can use the drawing select tool (keyboard short cut 1) to select any of that drawing's component drawing objects. You can hold down the shift key as you select drawing objects to create a temporary group of drawing objects. Once you have your desired drawing object selected you can use the "edit>copy drawing object" command to copy the drawing object. Next you need to select a new cell in the same or a different element in your exposure sheet. This often is confusing for people because if a cell doesn't exist for a frame location in an element and you select that frame location TBS will create a new cell and assign it to that selected frame as soon as you paste into the new cell or start to draw in the new cell. Just remember a frame is not a cell and cell is not a frame.

OK, so how do we paste a drawing object into our selected cell. We will use the "edit>paste drawing object" command. But this command is only available if the drawing view window has "focus". Which means we must click on the drawing view window to give it "focus" then we can execute the paste action for the copied drawing object.