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once a month is the most popular phrase on the web.

once per month 115,000,000 results on the web	once a month 1123,000,000 results on the web More popular!
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More than once per month [3] The efficiency of the converter shall be tested at least once per month. The current dairy market situation allows reducing the number of individual invitations to tender to once per month. Such frequency should, however, never be less than once per month. it is audited by CFIA, or an agency approved by CFIA, at least once per month and it has been concluded that it complies with the requirements of this Annex; On exit, once per month one health inspection; in those farms where the tests referred to in (i) have produced negative results, the health inspections shall continue once per month until the protection zone is withdrawn in accordance with point I..2.1(c); BAT is to use ISO 5667 for water sampling and to monitor the emissions to water at the point where the emission leaves the installation at least once per month [15] and in accordance with EN standards.	Probably reads e The Commission publishes this rate electronically once a month. And my daughter comes once a month. Village council meets once a month. mail once a month. Post these, once a month, without a return address. Same name as someone who stayed at the Republican Hotel once a month for ten years. It should be checked at least once a month.

once per month

115,000,000 results on the web

once a month

1123,000,000 results on the web

More popular!

More than once per month [3]
The efficiency of the converter shall be tested at least once per month.
The current dairy market situation allows reducing the number of individual invitations to tender to once per month.
Such frequency should, however, never be less than once per month.
it is audited by CFIA, or an agency approved by CFIA, at least once per month and it has been concluded that it complies with the requirements of this Annex;
On exit, once per month
one health inspection; in those farms where the tests referred to in (i) have produced negative results, the health inspections shall continue once per month until the protection zone is withdrawn in accordance with point I..2.1(c);
BAT is to use ISO 5667 for water sampling and to monitor the emissions to water at the point where the emission leaves the installation at least once per month [15] and in accordance with EN standards.

Probably reads e
The Commission publishes this rate electronically once a month.
And my daughter comes once a month.
Village council meets once a month.
mail once a month.
Post these, once a month, without a return address.
Same name as someone who stayed at the Republican Hotel once a month for ten years.
It should be checked at least once a month.