Why do I have to hit tab twice to Alt+Tab away from Microsoft Excel?

In Excel 2010 and below,

• Go to "Excel Options" → "Advanced" → "Display",
• Uncheck "Show all windows in the Taskbar",
• Click "OK" to close the Options window.

Now Alt+Tab should work fine again.

spioter says

[With] the [above] solution …, you will no longer be able to Alt+Tab between Excel documents [workbooks] – Alt+Tab will only "see" the last active Excel doc.  Work-around: with Excel active, Ctrl+Tab will switch between documents.

To revert this to the old behavior, simply go back and check the option again.

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This option does not exist in Excel 2013.  teylyn says

In the 2013 version, each Excel file shows in its own window.  This is one major change to previous versions.  Each window has its own ribbon and it is no longer possible to show several Excel files in the same window.

My workaround for this problem is to use the old Windows XP Alt + Tab UI, following the steps described in the answer to this Microsoft support thread:

1. Open Registry Editor (Win+R followed by regedit)
2. Go to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer
3. Create a new 32-bit DWORD value named AltTabSettings and set it to 1
4. Sign out and sign back in again (or restart)

Here is what the old Alt+Tab UI looks like. When Alt + Tabbing from the Excel window, the previous application is selected after the first Alt + Tab!

The old UI is a lot more limited, it doesn't show thumbnails for the open windows, and you can't click to select windows. But since I frequently Alt + Tab between different Excel windows, and also between Excel windows and other application windows, I'd prefer sticking with the older UI over having to deal with the Alt + Tab + Tab.