Lookups (including partylist and customer) cannot be copy pasted. I have encountered couple of scenarios where I needed this capability. I have developed these bookmarklets to solve this issue.

Copy Lookup

Select and the drag the code below to the bookmark bar.

javascript:(function(){let contentPanels=Array.from(document.querySelectorAll('iframe')).filter(function(d){return d.style.visibility!=='hidden'});if(contentPanels&&contentPanels.length>0){let Xrm=contentPanels[0].contentWindow.Xrm;let currentControl=Xrm.Page.ui.getCurrentControl();if(currentControl&¤tControl.getControlType()==='lookup'){let currentLookup=currentControl.getAttribute().getValue();if(currentLookup){let serialisedLookupValue=JSON.stringify(currentLookup.map(x=>{let c={};({id:c.id,name:c.name,type:c.type,typename:c.typename,entityType:c.entityType}=x);return c;}));sessionStorage.setItem('ryr_serialisedLookup',serialisedLookupValue);alert('Lookup copied. Ready to paste');}}else{alert('No field has been selected or the currently selected field is not a lookup');}}else{alert('Entity form not detected');}})();void 0;

Source:

(function () {
	let contentPanels = Array.from(document.querySelectorAll('iframe')).filter(function (d) {
			return d.style.visibility !== 'hidden'
		});
	if (contentPanels && contentPanels.length > 0) {
		let Xrm = contentPanels[0].contentWindow.Xrm;
		let currentControl = Xrm.Page.ui.getCurrentControl();
		if (currentControl && currentControl.getControlType() === 'lookup') {
			let currentLookup = currentControl.getAttribute().getValue();
			if (currentLookup) {
				let serialisedLookupValue = JSON.stringify(
						currentLookup.map(x =  > {
								let c = {};
								({
									id : c.id,
									name : c.name,
									type : c.type,
									typename : c.typename,
									entityType : c.entityType
								}
										 = x);
								return c;
							}));
				sessionStorage.setItem('ryr_serialisedLookup', serialisedLookupValue);
				alert('Lookup copied. Ready to paste');
			}
		} else {
			alert('No field has been selected or the currently selected field is not a lookup');
		}
	} else {
		alert('Entity form not detected');
	}
})();

Paste Lookup

Select and the drag the code below to the bookmark bar.

javascript:(function(){let contentPanels=Array.from(document.querySelectorAll('iframe')).filter(function(d){return d.style.visibility!=='hidden'});if(contentPanels&&contentPanels.length>0){let Xrm=contentPanels[0].contentWindow.Xrm;let currentControl=Xrm.Page.ui.getCurrentControl();if(currentControl&¤tControl.getControlType()==='lookup'){let currentLookup=currentControl.getAttribute();let copiedLookupValue=sessionStorage.getItem('ryr_serialisedLookup');if(copiedLookupValue){currentLookup.setValue(JSON.parse(copiedLookupValue));} else{alert('Please select a lookup to copy first before pasting');}}else{alert('No field has been selected or the currently selected field is not a lookup');}}else{alert('Entity form not detected');}})();void 0;

Source:

(function () {
	let contentPanels = Array.from(document.querySelectorAll('iframe')).filter(function (d) {
			return d.style.visibility !== 'hidden'
		});
	if (contentPanels && contentPanels.length > 0) {
		let Xrm = contentPanels[0].contentWindow.Xrm;
		let currentControl = Xrm.Page.ui.getCurrentControl();
		if (currentControl && currentControl.getControlType() === 'lookup') {
			let currentLookup = currentControl.getAttribute();
			let copiedLookupValue = sessionStorage.getItem('ryr_serialisedLookup');
			if(copiedLookupValue){
				currentLookup.setValue(JSON.parse(copiedLookupValue));
			}
			else{
				alert('Please select a lookup to copy first before pasting');
			}
		} else {
			alert('No field has been selected or the currently selected field is not a lookup');
		}
	} else {
		alert('Entity form not detected');
	}
})();

Instructions

1. Select the lookup field to be copied. The lookup will usually be highlighted with a blue background when it is selected
2. Run the Copy Lookup bookmarklet. You will get a alert message confirming that the lookup has been copied
3. Select the lookup field that is the target of the paste. The lookup has to be of the same type i.e. if the copied lookup was a contact lookup, the target lookup also has to be a contact lookup
4. Run the Paste Lookup bookmarklet
5. Save the record

I have tested these bookmarklets only in Chrome 53 and Firefox 50.