I reproduced it on M+, using a chrome tab in another window to trigger fetches (can be done through chrome:snippets-internals or by hitting the "more" button on the NTP)

Repro 1:

1. In NTP 1, clear all the articles
2. In NTP 2, on the second window, hit the more button

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Repro 2:
1. Turn on airplane mode
2. Clear all chrome data
3. Open Chrome, pass the FRE to get to NTP1
4. Open a new NTP2 in another window
5. Turn off airplane mode
6. In NTP2, hit the more button


For both repros:

Expected: articles added to NTP2, but not to NTP1, since I have interacted with it and can remember in which state it was.
Actual: articles added to both NTP1 and NTP2

Causes:
1. We only do the "has the user seen sugestions" check if there are suggestions at all. In that case there are no articles so we just append the suggestions.
2. If no suggestions have been seen because there are none, we have seen the whole section. But we still don't record that. So incoming suggestions will still be added.

Reproduced in Canary/59.0.3068.4