If a link specifies a target then such a link is opened in the tab or window with this name if you left-click such a link.
If you want to open the link in a new tab then you can middle-click such link.

But isn't that different link?

example.com/index.php?id=1 example.com/index.php?id=2

Tab name is also different (as page title).

If you click a link on a web page then the HTML code can contain other attributes and if a target window is specified then it will be used if you use a default left-click. The actual URL doesn't matter.

Firefox also have a Switch to Tab feature in the location bar drop down list to prevent opening a new tab with the same URL. In such a case you can press Shift or Alt while an entry is highlighted to see the URL.

Are you the author of the page with the two links? To ensure that both launch in their own tab, make sure you've set target="_blank" in the link or use window.open(url, "_blank").

I aam the author of webpage where link is pointing, but I cannot check by myself if the links are having "_blank" parameter because that web page is java based.

But if will ask those programers.