This is due to a bug in the Amazon-Link plugin with the call to add_menu_page.
From the codex:
WARNING: if two menu items use the same position attribute, one of the items may be overwritten so that only one item displays! Risk of conflict can be reduced by using decimal instead of integer values, e.g. 63.3 instead of 63 (Note: Use quotes in code, IE ‘63.3’).
I’m sure that last bit in the () was not there when I wrote the plugin…
Will be fixed in amazon-link version 3.1.1.
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