Global label only appears once in the schematic

This check is a census, not a connectivity test. ERC walks every sheet, collects every global label, resolves any text variables in it, and counts how many times each resulting name occurs in the whole schematic. Every name that occurs exactly once gets one violation, placed at that label.

Nothing else is considered: not pins, not wires, not whether the net goes anywhere. Two global labels with the same name silence the check even when they sit on the same wire, because the name now "appears twice". Names are compared after variable expansion, and MISO and miso are different names.

The check exists because a global label's job is to connect places by name; a name used once connects nothing, which usually means its partner is missing or misspelled. It ships at Ignore severity in KiCad 9 and 10, because a single global label is also a legitimate way to name a net and nothing more. You only see this violation after raising it to Warning or Error in Schematic Setup → Electrical Rules → Violation Severity (under Connections).

This check is opt-in — Ignore is the shipped default — so the honest framing is the reverse of most ERC pages: when should you turn it on? Enable it (Schematic Setup → Electrical Rules → Violation Severity → Connections) if your house style says global labels exist only to form cross-sheet pairs. It will catch typo'd and orphaned labels that Label not connected to anything misses: a typo'd global label sitting on a valid wire with pins is connected — to everything except its partner.

Once enabled, excluding individual markers (right-click → Exclude this violation) is reasonable for the deliberate single-use labels — a net named for layout or simulation. If you find yourself excluding most of what it reports, the default Ignore was right for you all along.

• The census counts text, not connections. Two same-named global labels on the same wire satisfy the check while connecting nothing new. Don't silence a violation by stacking a second label next to the first; fix the actual missing end.
• A typo produces two violations, not one. If the count of single_global_label markers is even, look for pairs of almost-equal names before anything else.
• KiCad 9 also has a separate, connectivity-based check: "Global label not connected anywhere else in the schematic" (global_label_dangling, Warning by default). KiCad 10 dropped it; this census is what remains, so a KiCad 10 project wanting label hygiene has to enable it.

[single_global_label]: Global label only appears once in the schematic
    ; warning
    @(116.84 mm, 101.60 mm): Global Label 'SPI_MISO'