Logging macros don't handle repeated keys

MWE:

```
julia> using Logging

julia> @info "foo" a=1 a=2
ERROR: syntax: field name "a" repeated in named tuple around logging/logging.jl:374
Stacktrace:
 [1] top-level scope
   @ REPL[3]:1
```

This should be explicitly checked for & give a better error message, instead of leaking a syntax error with hygiene issues (note that the location of the "syntax error" is in `logging/logging.jl`).