It is very common for researchers to present their work both at a conference and as a journal submission, and this is generally not considered duplicate submission. However, in such cases the conference presentation usually occurs before the journal submission, and only preliminary results are shared at the conference. After the conference, the study is built upon further and the complete findings are submitted as a journal article. Typically, the journal article should contain at least 30% original material that has not been presented before.
It is uncommon for the conference presentation to occur after the journal submission, and this could be considered duplicate submission, depending on the field of study. I'm not sure about the norm in your field of study, but both the journal and the conference are likely to have specific guidelines regarding duplicate submission and you should consult these guidelines carefully.
Since your paper has already been accepted by the journal, you should first write to the journal editor and ask if it's acceptable to submit the findings at the conference. A lot will also depend on whether copyright has been transferred to the journal at the time of acceptance. The journal might permit you to present the study at the conference, but might require you to present only an overview or an excerpt of the findings, and not the study in its entirety.
If you do get the journal's permission, your cover letter for the conference submission would have to disclose the fact that your study has been accepted by X journal and the expected date of publication. Finally, if your submission is accepted for the conference, your poster should also contain a footnote with this information.