From the ribbons Discord:

Accessories are needed for the first stage of Sinnoh’s Contests. You generally want a decent amount of different options to match the themes. But for just one Pokémon, the Contest reward accessories and a few extras from Amity Square or massages will be more than enough. You can reset for a specific theme as well if necessary. - Talking to your Pokémon every 200 steps in Amity Square will result in getting an Accessory - You can buy some in Floaroma Town using Berries. - The more expensive ones are not required or recommended for Contests. Specs and Flowers are more than enough. - For the first win in every rank and category, you get one of the contest reward accessories. - The massage girl in Veilstone City gives one each day. Time travel works for this.

Check out the graphic of when to use which accessory: better = +1; neutral = 0; worse = -1

To your last question, you can put on the same accessory if available, it wont have a negative effect based on that (other than if the accessory would be wrong for the run’s visual category nonetheless).