Thank you for the taking the time to file this bug! 

I wasn't able to reproduce this exactly as mentioned in the bug description, the Downloads trigger correctly the first time I tap on one of the download links. 

However, when I return to the custom tab and click on the download link again, it seems to trigger the download twice:
1. If I delete the previously downloaded file from Downloads before clicking on the download link the second time, the file begins to download AND a "Do you want to replace the existing file" infobar appears.
2. If I do not delete the previously downloaded file from Downloads before clicking on the download link the second time, TWO "Do you want to replace the existing file" infobars appear.

Notes:
- This happens only in a custom tab, not in a regular Chrome tab.
- I have not been able to reproduce this on other websites with pdf download links (e.g., irs.gov). reillyg@, are there any other websites where you see this behavior?

I find another random site and was also able to reproduce this. From this screenshot it looks like there are two dialogs overlapping.

http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=cc3200&fileType=pdf

Deleted: Screenshot_20161005-110248.png 60.9 KB

	Screenshot_20161005-110248.png 60.9 KB View Download

Hmm, I can actually repro this behavior in regular Chrome tabs too, not just in custom tabs.

regarding #2, is it possible the same link is clicked twice, probably due to a defect touch sensor on the device?

I found a touch events test pages (http://www.snappymaria.com/canvas/TouchEventTest.html) and it doesn't look like my device is generating duplicate events.

I see this for *all* downloads on m54 (tested with .mp4, .pdf, and .zip,  downloads). For example, try downloading any paper on arxiv.org:

1. Click to download PDF.
- "Do you want to replace the existing xxxx.xxxxxvx.pdf in <Download>?" infobar appears, even though the file has not previously been downloaded.
2. Click "Create new file" or "Replace file" (either works).
- PDF downloads (with toast and notification), and automatically launches in Drive PDF viewer once done.
- Only a single copy is listed in the Downloads app.
- Both Drive PDF viewer and the Downloads app show the original filename (i.e. it makes no difference whether you click "Create new file" or "Replace file").

The same thing happens for .zip or .mp4 downloads, except that instead of launching the Drive PDF Viewer, an "xxxxxxxxxx.{zip,mp4} downloaded [Open]" snackbar appears briefly.

I'm using Chrome Beta 54.0.2840.50 (NMF06B on both Nexus 6P and Pixel XL), in normal (non-custom) tabs.

Raising priority and adding ReleaseBlock-Stable since this is confusing and clearly broken UI, so we should do our best to avoid shipping it to stable, even if at this stage it may be too late. Jon, could you recommend someone to look into this please? Thanks!

This is almost certainly a dupe of  issue 641790  which should be fixed on the latest M54 release builds (fix CL only merged 3 days ago, not released to play store) - can you test on 54.0.2840.58 and let me know if it repros?

Ah good, yes this is indeed a duplicate.