most of our searches have mainline true which by-product gives us only one result for each order, however, if i want to access the item info it returns 4 results for one sales order how can I still get item data but only one line per order

If there are more than one line then it does not make sense to have a single line per transaction. If you are getting more lines even though there is a single, it is likely from shipping/tax. You can set

shipping line

and

tax line

to false to remove them.

thanx