I am implementing Google Analytics (via GTM) on multiple ecommerce sites. I need to record transactions to the client's google analytics account as well as to our single master analytics account, which accrues data for multiple sites.
I am wondering if there will be any issues sending duplicate transaction ids to our master google analytics's account i.e. if an order is placed on two different client sites and they happen to both be the same transaction id. Would Google Analytics recognise these as two separate transactions or would one overwrite the other?
If you send two identical transaction ids to the same account Google Analytics will add the second transaction to the first and show products for both under the same transaction id (before refund data upload became available that was actually used to cancel out unwanted transations using negative values for revenue).
However internally they will be counted as two transactions (i.e. in calculations for E-Commerce-Conversionrate etc.). Also if you set a time based second dimension (hour of day, minute index or the like) or e.g. by hostname you will see that Google untangles both transactions (so they appear added up only for timeframes that encompasses both).
To avoid this I would recommend that you use an advanced filter to add the hostname to the transaction id.