cmd/tailscale/cli: prevent all dup flags, not just strings

The earlier #15534 prevent some dup string flags. This does it for all
flag types.

Updates #6813

Change-Id: Iec2871448394ea9a5b604310bdbf7b499434bf01
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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