Each child in a list should have a unique "key" prop.

[ishak52]

Each child in the categories list should have a unique "key" prop.
in emarket/client/src/components/Category/index.js file

this.state.result.map(e => (
          <Link to={'/shop/'+e.id }>
            <div onClick={this.props.onClick}>
              <img className="grid-item" src={e.img} />
              <p className="category">{e.name}</p>
            </div>
            </Link>

Add key={e.id} to <Link to={'/shop/'+e.id }> to solve the problem.

Same thing for
/emarket/client/src/components/Product_card/index.js file

myProducts.map(e=>(
  <Link to={ '/product/' + e.id }><div className="cards">
  <div className="product-card-back" >
      <div className="product-card" >
      <img className="product-img" src={e.img} alt="product-img"/>
      <p className="product-name">{e.name}</p>
      <p className="product-price">₪ {e.price}</p>
    </div>
  </div>
  </div>
  </Link>

To solve this add key={e.id} to <Link to={ '/product/' + e.id }>