Error Parsing Duplicated Single-Value Flag

### Describe the bug

If a single-value flag is occasionally duplicated, it's interpreted incorrectly.

### Expected behavior

The system should check for a duplicated single-value flag and throw an error before launching the node.

### Steps to reproduce

Run a node locally with this command:
```
./lodestar beacon --dataDir ~/data/fusaka-devnet-4/data --paramsFile ~/data/fusaka-devnet-4/config.yaml --genesisStateFile ~/data/fusaka-devnet-4/genesis.ssz --bootnodes=$(cat ~/data/fusaka-devnet-4/bootstrap_nodes.txt | awk '{print $0}' | paste -sd ",") --execution.engineMock --eth1 false --checkpointSyncUrl https://checkpoint-sync.fusaka-devnet-4.ethpandaops.io --checkpointSyncUrl https://checkpoint-sync.fusaka-devnet-4.ethpandaops.io
```

The error:
```
Fetching checkpoint state checkpointSyncUrl=https://checkpoint-sync.fusaka-devnet-4.ethpandaops.io,https
✖ Error: Unable to fetch weak subjectivity state: HttpClient.urls[0] is empty or undefined: undefined
```

### Additional context

_No response_

### Operating system

Linux

### Lodestar version or commit hash

v.1.33.0