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Are you about to buy something you already own?

Every Microsoft 365 comparison tool answers “which plan should I buy”. This one answers the question people actually lose money on: is the add-on in front of you already inside the plan you hold? Pick your plan, answer three questions about the estate, and read 15 verdicts, each one sourced to Microsoft.

About the estate

Nothing is sent anywhere. This page makes no network request and stores nothing; the rules run in your browser. It names no price and calculates no saving, because it has never seen your tenant.

Result

Of 15 add-ons checked, 8 you already hold, 1 cannot be used, 1 has nothing to act on, and 5 would genuinely add something.

How it reaches a verdict

The rule base is not a research project of its own. It is what fell out of writing the ITSailor marketplace product pages: for each product we had to establish what the plans above it already include, what it needs to work at all, and where its ceiling sits. Those three facts are exactly what decides whether a purchase is a duplicate.

So every verdict here carries two links. One goes to the Microsoft page the rule was read from. The other goes to the ITSailor page that documents the same boundary in prose, so you can check the reasoning rather than take a coloured label on trust.

Where Microsoft has not published something clear enough to settle a pair, the answer is not settled by the documentation, and it stays that way. Guessing in either direction would make the tool feel more useful and be worth less.

Questions
How is this different from a Microsoft 365 plan comparison table?
A comparison table runs in the other direction. It tells you what a plan contains, so you can pick one. This tool starts from the plan you already hold and tells you whether the add-on in front of you is already inside it. That is the direction people lose money in, and almost nothing on the web answers it.
Where do the rules come from?
Every rule cites a first-party Microsoft page, linked next to each result. The rule base is a by-product of writing the ITSailor marketplace product pages, so each verdict also links to the page that documents the same boundary in prose.
Why does it never show a price or a saving?
It has never seen your tenant. A saving figure would be a measured claim about an estate this code cannot observe, so the tool reports inclusion, prerequisites and applicability only, and leaves the arithmetic to you.
What does "not settled by the documentation" mean?
That Microsoft has not published something clear enough for the pair you asked about. Rather than guess in either direction, the tool says so and tells you to confirm against your own licence assignment before ordering.
Does anything leave my browser?
No. The page makes no network request when you change an answer, stores nothing, and asks for no email address. The evaluation is a pure function running locally.
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