Identify Duplicate Transactions in Google Analytics
Duplicate transactions can prove themselves to be a royal pain, skew your marketing efforts, and reduce the trustworthiness the whole team have in the data and report from Google Analytics. Whilst it's not the easiest Analytics issue to fix in the world (although absolutely achievable), it is quite easy to find out if your site(s) are impacted by creating the following report.
Go to Analytics & select the view you're working with > Customisation > Custom Reports > click to create a new report and call it "Duped Transactions". Keep the type set to the default of "Explorer", for metric group select "Transactions" as a metric, and under Dimension Drilldowns select "Transaction ID" and save the report. It should look like the following.
Once you click save on the report it will direct you to the contents of the data, where you can set the date range to explore the date range you're interested. Click the "Advanced" link and set to "Include" "Transactions" with a quantity that is "Greater Than" "1", so that your filters look like the below.
Once you click "Apply" it will filter through the data set and return all transaction IDs that have more than one transactions associated with it; allowing you to dig deeper into the data to identify any issues - specifically any multiple counting of product quantity and revenue. This isn't typically a dataLayer or configuration issue, but has often been identified as multiple views of the confirmation / success page, or the more pesky - local caching of the page.