Such practices are known as dual representation which is not allowed under the Estate Agents Act. If found guilty, the agent can be fined $25,000 or to imprisonment for a term not exceeding 12 months or to both. You may refer to CEA's link here: https://www.cea.gov.sg/docs/default-source/legislation-guideliness/estateagentsrevised.pdf
You may wish to lodge a complaint with the CEA on such agents.
Hope this answers your question.