Joining Tables With Multiple Criteria

I have a straight-forward select query to show work orders for a particular customer as below. I want to add a field value from another table, deltickitem diwhich contains contract records. I need to include the field di.weekchg to show the weekly hire rate, but the joined query must ensure that the both the contract number matches that in the original select and that the item number matches that in the actual select. Additionally, there is the problem that the item can appear more than once in the deltickitem table against a particular contract (if item has been off-hired and then re-hired on the same contract number) - in this case the query must select the record with the highest di.counter number, which I haven't worked out how to put in my query.

This is my basic code, but I keep ending up with duplicate work order lines in my result set.

Select wh.worknumber, wh.custnum, wh.contract, wh.sitename, wh.itemcode, wh.regnum, m.name, di.weekchg,
wh.date_created, wh.task_descr, wh.actual_labour_sale+wh.actual_parts_sale as [Repair Cost]
From worksorderhdr wh Left Join
inventory iv On iv.item = wh.itemcode inner Join
models m On m.id = iv.model_id left join deltickitem di on di.dticket = wh.contract
where wh.custnum = 'BARRATNE' and wh.rejected <> 1 and wh.charge_to_cust = 1
order by wh.date_created