Spring boot - NonUniqueResultException: query did not return a unique result: error

Question

I have 1 entities, Post and FileUploads where a post can have many FileUploads (to add images to the post/advert)

I am able to send a request for the first time and the data is persisted properly in the database, the issue is when I try to create another Post with the FileUploads that I get this error.

I'm not sure what's causing it. Only the first post with the FileUploads gets persisted, after that only the posts get persisted without the FileUploads.

Post.java

@Entity
public class Post {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NotBlank(message = "Required")
    @Size(max=45, message = "Maximum of 45 letters")
    @Column()
    private String title;

    @NotBlank(message = "Required")
    @Size(min = 40, message = "Minimum of 40 characters")
    @Column()
    private String description;

    @NotBlank(message = "Required")
    @Column()
    private String brand;

    @NotBlank(message = "Required")
    @Column()
    private String model;

    @NotNull(message = "Required")
    @Column()
    private Double price;

    @NotBlank(message = "Required")
    @Column()
    private String color;

    @NotNull(message = "Required")
    @Column()
    private Double kilometers;

    @NotBlank(message = "Required")
    @Column()
    private String bodyType;

    @NotBlank(message = "Required")
    @Column()
    private String fuelType;

    @NotBlank(message = "Required")
    @Column()
    private String transmission;

    @NotNull(message = "Required")
    @Column()
    private Integer year;

    @NotBlank(message = "Required")
    @Column()
    private String location;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, mappedBy = "post")
    private List<FileUpload> fileUploads = new ArrayList<>();

    @Column(name="post_creator_email")
    private String postCreatorEmail;

    @Column(name="post_creator_name")
    private String postCreatorName;

    public Post() {
    }

// getters and setters

---

FileUpload.java

@Entity
public class FileUpload {

    @Id
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    @GeneratedValue(generator = "uuid")
    private String fileId;

    @Column(name="name")
    private String name;

    @Column(name="type")
    private String type;

    @Column(name="file_uploader")
    private String fileUploader;

    @Column(name="image_count")
    private int imageCount;

    @Lob
    @JsonIgnore
    @Column(name="data")
    private byte[] data;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name="post_id")
    @JsonIgnore
    private Post post;

    public FileUpload() {

    }

    public FileUpload(String name, String type, String fileUploader, int imageCount, byte[] data) {
        this.name = name;
        this.type = type;
        this.fileUploader = fileUploader;
        this.imageCount = imageCount;
        this.data = data;
    }
}

---

FileUploadServiceImpl.java (method to store images)

    @Override
    public void uploadPostImages(List<FileUpload> fileUploadList, String emailAddress) {
        Post thePost = postRepository.findPostByPostCreatorEmail(emailAddress);

        if(thePost == null) {
            System.out.println("post is null");
            throw new PostNotFoundException("No post found for email " + emailAddress);
        }


        for(FileUpload fileUpload : fileUploadList) {
            fileUpload.setPost(thePost);
        }
        thePost.setFileUploads(fileUploadList);
        postRepository.save(thePost);

    }

---

PostServiceImpl.java (method to create post)

 @Override
    public void createOrUpdatePost(String post, String emailAddress) {
        Post thePost = new Post();

        if (thePost.getId() != null) { //checks if a post with ID already exists
            thePost = postRepository.findPostById(thePost.getId());

            if (thePost != null && !(thePost.getPostCreatorEmail().equals(emailAddress))) {
                throw new PostNotFoundException("Post not found in your account");
            } else if (thePost == null) {
                throw new PostNotFoundException("Post does not exist");
            }
        }

        // if a post does not exist, CREATE
        try {
            User user = userRepository.findUserByEmailAddress(emailAddress);
            System.out.println("looking for user with email: " + emailAddress);

            ObjectMapper objectMapper = new ObjectMapper();
            thePost = objectMapper.readValue(post, Post.class);

            user.setTotalAds(++count);
            thePost.setPostCreatorName(user.getFullName());
            thePost.setPostCreatorEmail(emailAddress);
            thePost.setUser(user);
            userRepository.save(user);
            postRepository.save(thePost);
        } catch (Exception e) {
            throw new PostAlreadyExistsException("Post with title " + thePost.getTitle() + " already exists");
        }

    }

---

PostController.java

    @PostMapping("/create")
    public ResponseEntity<?> createPost(@RequestPart("files") MultipartFile[] files, @Valid @RequestPart String post,  BindingResult result, Principal principal) throws IOException {
        ResponseEntity<?> errorMap = errorValidationService.validationService(result);
        if(errorMap != null) return errorMap;

        String message = "";

        try {
            List<FileUpload> fileList = new ArrayList<>();

            for (MultipartFile file : files) {
                String fileContentType = file.getContentType();
                String fileName = file.getOriginalFilename();
                FileUpload fileUpload = new FileUpload(fileName, fileContentType, principal.getName(),++imageCount, file.getBytes());

                fileList.add(fileUpload);
            }

            postService.createOrUpdatePost(post, principal.getName());
            fileUploadService.uploadPostImages(fileList, principal.getName());

            message = "Uploaded the files successfully";
            return ResponseEntity.status(HttpStatus.OK).body(new ApiResponse(message, true));
        } catch(Exception e) {
            message = e.getMessage();
            return ResponseEntity.status(HttpStatus.EXPECTATION_FAILED).body(new ApiResponse(message, false));
        }

I am making sure to change all the fields on the second request but i'm still having the same error message no matter what..