iTunes downloading ios update multiple times

This is a continuation, in a way, of Pietmann's query. I have a very slow internet connection (ethernet/dsl) and need to update my iPad iOS. I had the same problem when I updated my iPhone and was ready to try recovery mode when suddenly it worked. SO what is happening is that I have attached my iPad to my iMac to update to 13.5.1. I backed up my iPad, then asked for a download only, since I knew it would take a long time and I wasn't ready to leave it attached "forever". The update downloaded; I verified it by looking in the folder. Then I tried to update and it asked me to back up my purchases, which I did. Then I asked again for an update and it contacted the server and started to download the update AGAIN, when it is already sitting in the folder where it is supposed to be. I paused the download and tried the update again. Same thing happened--another download. Tried again, another download, which I am now letting go in hopes that it will simply update when it finishes in seven hours. There is obviously something wrong in iTunes, since it is not checking for an existing update before updating. Can anyone help? btw, I am on Mojave (10.14.4) if that helps. I found Pietmann's question about the same thing and the response was honestly not an answer at all. Not all of us have the capability of getting to a fast internet connection and I'm not at all sure that would fix the problem.