add validator for already existing owner address

src/routes/safe/components/Settings/ManageOwners/AddOwnerModal/index.jsx

@@ -123,7 +123,9 @@ const AddOwner = ({
               paperClassName={classes.biggerModalWindow}
             >
               <React.Fragment>
-                {activeScreen === 'selectOwner' && <OwnerForm onClose={onClose} onSubmit={ownerSubmitted} />}
+                {activeScreen === 'selectOwner' && (
+                  <OwnerForm onClose={onClose} onSubmit={ownerSubmitted} owners={owners} />
+                )}
                 {activeScreen === 'selectThreshold' && (
                   <ThresholdForm
                     onClose={onClose}

src/routes/safe/components/Settings/ManageOwners/AddOwnerModal/screens/OwnerForm/index.jsx

@@ -13,6 +13,7 @@ import Block from '~/components/layout/Block'
 import Hairline from '~/components/layout/Hairline'
 import Field from '~/components/forms/Field'
 import TextField from '~/components/forms/TextField'
+import { type Owner } from '~/routes/safe/store/models/owner'
 import {
   composeValidators,
   required,
@@ -26,13 +27,16 @@ type Props = {
   onClose: () => void,
   classes: Object,
   onSubmit: Function,
-  owners: List<string>,
+  owners: List<Owner>,
 }
 
-const OwnerForm = ({ classes, onClose, onSubmit }: Props) => {
+const OwnerForm = ({
+  classes, onClose, onSubmit, owners,
+}: Props) => {
   const handleSubmit = (values) => {
     onSubmit(values)
   }
+  const ownerDoesntExist = uniqueAddress(owners.map(o => o.address))
 
   return (
     <React.Fragment>
@@ -72,7 +76,7 @@ const OwnerForm = ({ classes, onClose, onSubmit }: Props) => {
                     name="ownerAddress"
                     component={TextField}
                     type="text"
-                    validate={composeValidators(required, mustBeEthereumAddress)}
+                    validate={composeValidators(required, mustBeEthereumAddress, ownerDoesntExist)}
                     placeholder="Owner address*"
                     text="Owner address*"
                     className={classes.addressInput}