fix: reject duplicate flag names

[deepakganesh78]

What type of PR is this?

• bug

What this PR does / why we need it:

• Rejects genuine duplicate user-defined local flag names and aliases on a command, so ambiguous command configurations fail early.
• Makes built-in help flag installation name-aware: if a user flag already claims help or h, the auto-added help flag is not appended.
• Keeps the existing name-aware version flag behavior unchanged.

Before this change, a command with a user-defined &BoolFlag{Name: "help"} received the built-in help flag too, leaving two flags named help and duplicated help output. After this change, the user-defined help flag wins, overriding the help flag continues to work, --help still parses without error, and only one help flag is present.

Which issue(s) this PR fixes:

Fixes #2174

Special notes for your reviewer:

Compatibility note: user-defined overrides of the built-in help flag continue to work. Only configurations where the user's own Flags slice contains duplicate names/aliases now return an error during Run.

Testing

• Regression checks: go test -run "TestDuplicateFlagNamesAreRejected|TestUserDefinedHelpFlagOverridesBuiltin" -count=1 -v fails on unmodified source (TestDuplicateFlagNamesAreRejected gets nil errors; TestUserDefinedHelpFlagOverridesBuiltin finds 2 help flags) and passes with this fix.
• go build ./... ✅
• go vet ./... ✅
• gofmt -l . ✅ empty
• go test ./... ✅
• go run ./scripts/build.go generate ✅
• go run ./scripts/build.go vet ✅
• go run ./scripts/build.go check-binary-size ✅ (1.8MB current size; target 1.5MB–2.2MB)
• go run ./scripts/build.go v3diff ✅ (with Git diff on PATH)

Release Notes

Reject command configurations with duplicate user-defined flag names or aliases.