User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:66.0) Gecko/20100101 Firefox/66.0

Steps to reproduce:

Open a webpage in a tab. Go to a new tab (blank or with a webpage). In the address bar, type the name or url of the first tab such that it is highlighted. Attempt to open that URL in a new tab by pressing Alt+Enter.

Actual results:

The current tab is overwritten.

Expected results:

The URL should open in a new tab regardless of any existing tabs with that URL. The expected behaviour works fine with URLs which are not open in existing tabs.

Could you please clarify what you mean by "such that it is highlighted"? Do you actually select the switch-to-tab entry from the popup?

The highlighting part is somewhat extraneous information. I just mean select the URL of the first tab. For example how you can start typing a name or URL and Firefox provides a list of closest matches. Then use the arrow keys or mouse-over to select one of them (highlighted in light blue for me).

I don't select 'switch to tab'. I'm trying to open a page already open in another tab in a new tab using Alt+Enter or Ctrl+Left Click

Also interesting how the modifier to open a link in new tab is different between the mouse and keyboard for the same thing: Ctrl and Alt.

I just realized Ctrl+Click has the same problematic behaviour as Alt+Enter but there's already existing bug 1536756 open for the Ctrl+Click issue. Thus I'd imagine this bug is a duplicate but describing a slightly different symptom.

You say you used arrow keys to select one of the results. is that entry a "Switch to tab" entry? That is usually indicated with a tab icon on the left and the "switch to tab" text on the right, after the url.

I'm asking because those entries are special, and ALT on those entries mean "Open in this tab instead fo switching to that tab", that is confusing, I admit, but it's the current situation.

Ah I see what you mean by that now, it's supposed to be a feature. But at the expense of an existing behaviour. It's confusing and frustrating because it makes the behaviour inconsistent. Could there be a setting in about:config to toggle this?

This is pretty much the same as bug 1536756: shift, alt and ctrl are all override for switch to tab, a solution should be found there, if any.