Because the URL in this line
property inboxURL : "tinderbox://demo/?view=map+select=1492694214;"
tells Tinderbox to open that document and open that tab. Tinderbox does not parse the environment and decide “aha, I have that tab already open and ready”. It just creates the tab regardless. If you paste your local version of that URL into a browser and hit refresh repeatedly, you’d get the same result – new tab + new tab + new tab …
btw … now that you’ve joined the KM crew you’ll find lots of reasons to wonder why you never did that before … and you can eventually replace the script, above, with a KM macro.