fix(pulse): refuse a reused --target name

Description

The pitch.

One of nine independent PRs from a correctness audit of `main` at `df0b5f1`. Each merges cleanly with the others in any order; together they pass 908 tests locally on Python 3.11 and 3.14 (CI covers 3.11 and 3.12).

Commits: `a415aaf`

### Problem

`nandatown pulse --target svc=A --target svc=B` silently probed only B and
exited 0.

### Change

A reused name is refused with exit 2 and a message, before any probe or database
write.

### Compatibility

Distinct names are unchanged.

### Verification

- Full suite on the branch: 818 passed.
- A test with counting localhost servers checks exit 2, the message, zero hits
  and no database. It fails on `df0b5f1`.

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