The phrase "alert once again" is correct and usable in written English.
It can be used when notifying someone about a situation or issue that has occurred previously and is happening again.
Example: "I would like to alert once again that the deadline for submissions is approaching quickly."
Alternatives: "notify once more" or "remind again".
Today the campaign launched an advertising campaign to alert voters once again to this attack on their voting rights and to this power grab by the NRA and the politicians who do their bidding.
Interpol was alerted, but once again Biggs was one step ahead, fleeing to Brazil, with his long-suffering wife's blessing.
Both messages alerted her that — once again — her information had been compromised.
This campaign will once again alert consumers to the "anti-competitive" nature of the proposed tie-up, which Virgin hopes will then indirectly put pressure on American antitrust regulators.
The dog once again "alerted" and the officer opened the trunk.
Ames once again alerted the president, who dispatched Gen. Philip Sheridan to restore order to the entire Gulf region.
Real Madrid manager Jose Mourinho, 49, has once again alerted top Premier League clubs, including his former employers Chelsea, to his potential availability in the summer by reiterating a desire to work in England again.
The recent discovery of "flock worker's lung" due to synthetic fibers once again alerts us to emerging diseases associated with new technologies.
But in late October, No Labels announced that it would organize get out the vote efforts from Oct. 31 through Election Day on Gardner's behalf, once again without alerting Manchin.
After the scare of 1457, when French troops successfully attacked the town of Sandwich on the south coast, Southampton Castle was once again put on alert, and one of Henry VI's more "dependable captains", Nicholas Carew, was given control of the castle for life.
