If you are using a keyword more than once because you are using keyphrases like i.e "blue widgets, red widgets, yellow widgets" you should better use commas, only because it can trigger keywords repetition/stuffing filters, or it can come to a spam team manual review, for example by Yahoo.

So if you do not use commas, it will look like this: "blue widgets red widgets yellow widgets".

And that is just a risk with no benefit!

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